SmilesTokenizer Instance can not be created
## 🐛 Bug

I wanted to use the SmilesTokenizer from deepchem as it is explained in [the docs](https://deepchem.readthedocs.io/en/2.4.0/api_reference/tokenizers.html#smilestokenizer). However, i'm not able to create an instance of the SmilesTokenizer due to the following error:
```
Traceback(most recent call last):
File "/tmp/pycharm_projects/spielwiese/main.py", line 19, in <module>
    tokenizer = SmilesTokenizer(vocab_path)
File "/root/miniconda3/lib/python3.8/site-packages/deepchem/feat/smiles_tokenizer.py", line 89, in __init__
    self.max_len_single_sentence = self.max_len - 2
AttributeError: 'SmilesTokenizer' object has no attribute 'max_len
```

This seems to be caused by incompabilities between the versions of deepchem and transformers as it was already mentioned [here.](https://github.com/huggingface/transformers/issues/8739) However, the fix "changing max_len to model_max_length" has not been applied to deepchem yet. 

This would be nice for using the latest versions of all packages involved.

* Python version: 3.8.5
* DeepChem version: 2.5.0
* Transformers version: 4.6.0
* PyTorch version (optional): 1.8.1


diff --git a/deepchem/feat/smiles_tokenizer.py b/deepchem/feat/smiles_tokenizer.py
index 233db6b44..8aa0663bb 100644
--- a/deepchem/feat/smiles_tokenizer.py
+++ b/deepchem/feat/smiles_tokenizer.py
@@ -86,8 +86,8 @@ class SmilesTokenizer(BertTokenizer):
 
     super().__init__(vocab_file, **kwargs)
     # take into account special tokens in max length
-    self.max_len_single_sentence = self.max_len - 2
-    self.max_len_sentences_pair = self.max_len - 3
+    self.max_len_single_sentence = self.model_max_length - 2
+    self.max_len_sentences_pair = self.model_max_length - 3
 
     if not os.path.isfile(vocab_file):
       raise ValueError(
@@ -98,7 +98,7 @@ class SmilesTokenizer(BertTokenizer):
     self.ids_to_tokens = collections.OrderedDict(
         [(ids, tok) for tok, ids in self.vocab.items()])
     self.basic_tokenizer = BasicSmilesTokenizer()
-    self.init_kwargs["max_len"] = self.max_len
+    self.init_kwargs["model_max_length"] = self.model_max_length
 
   @property
   def vocab_size(self):
['deepchem/feat/smiles_tokenizer.py']