BLEURT score calculation raises UnrecognizedFlagError
This happens when running the notebook on colab. The issue seems to be that colab populates sys.argv with arguments not handled by bleurt.

Running this before calling bleurt fixes it:
```
import sys
sys.argv = sys.argv[:1]
```

Not the most elegant solution. Perhaps it needs to be fixed in the bleurt code itself rather than huggingface?

This is the output of `print(sys.argv)` when running on colab:
```
['/usr/local/lib/python3.7/dist-packages/ipykernel_launcher.py', '-f', '/root/.local/share/jupyter/runtime/kernel-a857a78c-44d6-4b9d-b18a-030b858ee327.json']
```
Calling the `compute` method for **bleurt** metric fails with an `UnrecognizedFlagError` for `FLAGS.bleurt_batch_size`. 

My environment:
```
python==3.8.5
datasets==1.2.0
tensorflow==2.3.1
cudatoolkit==11.0.221
```

Test code for reproducing the error:
```
from datasets import load_metric
bleurt = load_metric('bleurt')
gen_text = "I am walking on the promenade today"
ref_text = "I am walking along the promenade on this sunny day"
bleurt.compute(predictions=[test_text], references=[test_text])
```

Error Output:
```
Using default BLEURT-Base checkpoint for sequence maximum length 128. You can use a bigger model for better results with e.g.: datasets.load_metric('bleurt', 'bleurt-large-512').
INFO:tensorflow:Reading checkpoint /home/ubuntu/.cache/huggingface/metrics/bleurt/default/downloads/extracted/9aee35580225730ac5422599f35c4986e4c49cafd08082123342b1019720dac4/bleurt-base-128.
INFO:tensorflow:Config file found, reading.
INFO:tensorflow:Will load checkpoint bert_custom
INFO:tensorflow:Performs basic checks...
INFO:tensorflow:... name:bert_custom
INFO:tensorflow:... vocab_file:vocab.txt
INFO:tensorflow:... bert_config_file:bert_config.json
INFO:tensorflow:... do_lower_case:True
INFO:tensorflow:... max_seq_length:128
INFO:tensorflow:Creating BLEURT scorer.
INFO:tensorflow:Loading model...
INFO:tensorflow:BLEURT initialized.
---------------------------------------------------------------------------
UnrecognizedFlagError                     Traceback (most recent call last)
<ipython-input-12-8b3f4322318a> in <module>
      2 gen_text = "I am walking on the promenade today"
      3 ref_text = "I am walking along the promenade on this sunny day"
----> 4 bleurt.compute(predictions=[gen_text], references=[ref_text])

~/anaconda3/envs/noved/lib/python3.8/site-packages/datasets/metric.py in compute(self, *args, **kwargs)
    396             references = self.data["references"]
    397             with temp_seed(self.seed):
--> 398                 output = self._compute(predictions=predictions, references=references, **kwargs)
    399 
    400             if self.buf_writer is not None:

~/.cache/huggingface/modules/datasets_modules/metrics/bleurt/b1de33e1cbbcb1dbe276c887efa1fad68c6aff913885108078fa1ad408908778/bleurt.py in _compute(self, predictions, references)
    103 
    104     def _compute(self, predictions, references):
--> 105         scores = self.scorer.score(references=references, candidates=predictions)
    106         return {"scores": scores}

~/anaconda3/envs/noved/lib/python3.8/site-packages/bleurt/score.py in score(self, references, candidates, batch_size)
    164     """
    165     if not batch_size:
--> 166       batch_size = FLAGS.bleurt_batch_size
    167 
    168     candidates, references = list(candidates), list(references)

~/anaconda3/envs/noved/lib/python3.8/site-packages/tensorflow/python/platform/flags.py in __getattr__(self, name)
     83     # a flag.
     84     if not wrapped.is_parsed():
---> 85       wrapped(_sys.argv)
     86     return wrapped.__getattr__(name)
     87 

~/anaconda3/envs/noved/lib/python3.8/site-packages/absl/flags/_flagvalues.py in __call__(self, argv, known_only)
    643     for name, value in unknown_flags:
    644       suggestions = _helpers.get_flag_suggestions(name, list(self))
--> 645       raise _exceptions.UnrecognizedFlagError(
    646           name, value, suggestions=suggestions)
    647 

UnrecognizedFlagError: Unknown command line flag 'f'
```

Possible Fix:
Modify `_compute` method https://github.com/huggingface/datasets/blob/7e64851a12263dc74d41c668167918484c8000ab/metrics/bleurt/bleurt.py#L104
to receive a `batch_size` argument, for example:
```
def _compute(self, predictions, references, batch_size=1):
    scores = self.scorer.score(references=references, candidates=predictions, batch_size=batch_size)
    return {"scores": scores}
```
BLEURT score calculation raises UnrecognizedFlagError 
 Calling the `compute` method for **bleurt** metric fails with an `UnrecognizedFlagError` for `FLAGS.bleurt_batch_size`. 

My environment:
```
python==3.8.5
datasets==1.2.0
tensorflow==2.3.1
cudatoolkit==11.0.221
```

Test code for reproducing the error:
```
from datasets import load_metric
bleurt = load_metric('bleurt')
gen_text = "I am walking on the promenade today"
ref_text = "I am walking along the promenade on this sunny day"
bleurt.compute(predictions=[test_text], references=[test_text])
```

Error Output:
```
Using default BLEURT-Base checkpoint for sequence maximum length 128. You can use a bigger model for better results with e.g.: datasets.load_metric('bleurt', 'bleurt-large-512').
INFO:tensorflow:Reading checkpoint /home/ubuntu/.cache/huggingface/metrics/bleurt/default/downloads/extracted/9aee35580225730ac5422599f35c4986e4c49cafd08082123342b1019720dac4/bleurt-base-128.
INFO:tensorflow:Config file found, reading.
INFO:tensorflow:Will load checkpoint bert_custom
INFO:tensorflow:Performs basic checks...
INFO:tensorflow:... name:bert_custom
INFO:tensorflow:... vocab_file:vocab.txt
INFO:tensorflow:... bert_config_file:bert_config.json
INFO:tensorflow:... do_lower_case:True
INFO:tensorflow:... max_seq_length:128
INFO:tensorflow:Creating BLEURT scorer.
INFO:tensorflow:Loading model...
INFO:tensorflow:BLEURT initialized.
---------------------------------------------------------------------------
UnrecognizedFlagError                     Traceback (most recent call last)
<ipython-input-12-8b3f4322318a> in <module>
      2 gen_text = "I am walking on the promenade today"
      3 ref_text = "I am walking along the promenade on this sunny day"
----> 4 bleurt.compute(predictions=[gen_text], references=[ref_text])

~/anaconda3/envs/noved/lib/python3.8/site-packages/datasets/metric.py in compute(self, *args, **kwargs)
    396             references = self.data["references"]
    397             with temp_seed(self.seed):
--> 398                 output = self._compute(predictions=predictions, references=references, **kwargs)
    399 
    400             if self.buf_writer is not None:

~/.cache/huggingface/modules/datasets_modules/metrics/bleurt/b1de33e1cbbcb1dbe276c887efa1fad68c6aff913885108078fa1ad408908778/bleurt.py in _compute(self, predictions, references)
    103 
    104     def _compute(self, predictions, references):
--> 105         scores = self.scorer.score(references=references, candidates=predictions)
    106         return {"scores": scores}

~/anaconda3/envs/noved/lib/python3.8/site-packages/bleurt/score.py in score(self, references, candidates, batch_size)
    164     """
    165     if not batch_size:
--> 166       batch_size = FLAGS.bleurt_batch_size
    167 
    168     candidates, references = list(candidates), list(references)

~/anaconda3/envs/noved/lib/python3.8/site-packages/tensorflow/python/platform/flags.py in __getattr__(self, name)
     83     # a flag.
     84     if not wrapped.is_parsed():
---> 85       wrapped(_sys.argv)
     86     return wrapped.__getattr__(name)
     87 

~/anaconda3/envs/noved/lib/python3.8/site-packages/absl/flags/_flagvalues.py in __call__(self, argv, known_only)
    643     for name, value in unknown_flags:
    644       suggestions = _helpers.get_flag_suggestions(name, list(self))
--> 645       raise _exceptions.UnrecognizedFlagError(
    646           name, value, suggestions=suggestions)
    647 

UnrecognizedFlagError: Unknown command line flag 'f'
```

Possible Fix:
Modify `_compute` method https://github.com/huggingface/datasets/blob/7e64851a12263dc74d41c668167918484c8000ab/metrics/bleurt/bleurt.py#L104
to receive a `batch_size` argument, for example:
```
def _compute(self, predictions, references, batch_size=1):
    scores = self.scorer.score(references=references, candidates=predictions, batch_size=batch_size)
    return {"scores": scores}
``` 
 This happens when running the notebook on colab. The issue seems to be that colab populates sys.argv with arguments not handled by bleurt.

Running this before calling bleurt fixes it:
```
import sys
sys.argv = sys.argv[:1]
```

Not the most elegant solution. Perhaps it needs to be fixed in the bleurt code itself rather than huggingface?

This is the output of `print(sys.argv)` when running on colab:
```
['/usr/local/lib/python3.7/dist-packages/ipykernel_launcher.py', '-f', '/root/.local/share/jupyter/runtime/kernel-a857a78c-44d6-4b9d-b18a-030b858ee327.json']
```