Bug Report: timestamp[ns] not recognized
Thanks for reporting !

You're right, `string_to_arrow` should be able to take `"timestamp[ns]"` as input and return the right pyarrow timestamp type.
Feel free to suggest a fix for `string_to_arrow` and open a PR if you want to contribute ! This would be very appreciated :)

To give you more context:

As you may know we define the features types of a dataset using the `Features` object in combination with feature types like `Value`. For example
```python
features = Features({
    "age": Value("int32")
})
```
However under the hood we are actually using pyarrow to store the data, and so we have a mapping between the feature types of `datasets` and the types of pyarrow.

For example, the `Value` feature types are created from a pyarrow type with `Value(str(pa_type))`.
However it looks like the conversion back to a pyarrow type doesn't work with `"timestamp[ns]"`.
This is the `string_to_arrow` function you highlighted that does this conversion, so we should fix that.


Repro:

```
from datasets import Dataset
import pandas as pd
import pyarrow

df = pd.DataFrame(pd.date_range("2018-01-01", periods=3, freq="H"))
pyarrow.Table.from_pandas(df)
Dataset.from_pandas(df)
# Throws ValueError: Neither timestamp[ns] nor timestamp[ns]_ seems to be a pyarrow data type.
```

The factory function seems to be just "timestamp": https://arrow.apache.org/docs/python/generated/pyarrow.timestamp.html#pyarrow.timestamp

It seems like https://github.com/huggingface/datasets/blob/master/src/datasets/features.py#L36-L43 could have a little bit of additional structure for handling these cases?  I'd be happy to take a shot at opening a PR if I could receive some guidance on whether parsing something like `timestamp[ns]` and resolving it to timestamp('ns') is the goal of this method.

Alternatively, if I'm using this incorrectly (e.g. is the expectation that we always provide a schema when timestamps are involved?), that would be very helpful to know as well!

```
$ pip list  # only the relevant libraries/versions
datasets                      1.2.1
pandas                        1.0.3
pyarrow                       3.0.0
```
Bug Report: timestamp[ns] not recognized 
 Repro:

```
from datasets import Dataset
import pandas as pd
import pyarrow

df = pd.DataFrame(pd.date_range("2018-01-01", periods=3, freq="H"))
pyarrow.Table.from_pandas(df)
Dataset.from_pandas(df)
# Throws ValueError: Neither timestamp[ns] nor timestamp[ns]_ seems to be a pyarrow data type.
```

The factory function seems to be just "timestamp": https://arrow.apache.org/docs/python/generated/pyarrow.timestamp.html#pyarrow.timestamp

It seems like https://github.com/huggingface/datasets/blob/master/src/datasets/features.py#L36-L43 could have a little bit of additional structure for handling these cases?  I'd be happy to take a shot at opening a PR if I could receive some guidance on whether parsing something like `timestamp[ns]` and resolving it to timestamp('ns') is the goal of this method.

Alternatively, if I'm using this incorrectly (e.g. is the expectation that we always provide a schema when timestamps are involved?), that would be very helpful to know as well!

```
$ pip list  # only the relevant libraries/versions
datasets                      1.2.1
pandas                        1.0.3
pyarrow                       3.0.0
``` 
 Thanks for reporting !

You're right, `string_to_arrow` should be able to take `"timestamp[ns]"` as input and return the right pyarrow timestamp type.
Feel free to suggest a fix for `string_to_arrow` and open a PR if you want to contribute ! This would be very appreciated :)

To give you more context:

As you may know we define the features types of a dataset using the `Features` object in combination with feature types like `Value`. For example
```python
features = Features({
    "age": Value("int32")
})
```
However under the hood we are actually using pyarrow to store the data, and so we have a mapping between the feature types of `datasets` and the types of pyarrow.

For example, the `Value` feature types are created from a pyarrow type with `Value(str(pa_type))`.
However it looks like the conversion back to a pyarrow type doesn't work with `"timestamp[ns]"`.
This is the `string_to_arrow` function you highlighted that does this conversion, so we should fix that.

