Cannot import transformers with TF version 2.1.0
The installation README says that transformers library requires Tensorflow version >2.0, but I can't seem to import the latest transformers 3.2 release even with Tensorflow v2.1. 

```
>>> import transformers
wandb: WARNING W&B installed but not logged in.  Run `wandb login` or set the WANDB_API_KEY env variable.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/amog/dev/ray/lib/python3.7/site-packages/transformers/__init__.py", line 121, in <module>
    from .pipelines import (
  File "/Users/amog/dev/ray/lib/python3.7/site-packages/transformers/pipelines.py", line 47, in <module>
    from .modeling_tf_auto import (
  File "/Users/amog/dev/ray/lib/python3.7/site-packages/transformers/modeling_tf_auto.py", line 45, in <module>
    from .modeling_tf_albert import (
  File "/Users/amog/dev/ray/lib/python3.7/site-packages/transformers/modeling_tf_albert.py", line 24, in <module>
    from .activations_tf import get_tf_activation
  File "/Users/amog/dev/ray/lib/python3.7/site-packages/transformers/activations_tf.py", line 53, in <module>
    "swish": tf.keras.activations.swish,
AttributeError: module 'tensorflow_core.python.keras.api._v2.keras.activations' has no attribute 'swish'
```

Upgrading to TF 2.2 works fine, but I think this should be made more clear in the docs.

cc @jplu @sgugger 
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.2.0
- Platform: Mac OS
- Python version: 3.7.7
- PyTorch version (GPU?):
- Tensorflow version (GPU?): 2.1.0. On CPU only.
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

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 TextGeneration: @TevenLeScao 
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 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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## Information

Model I am using (Bert, XLNet ...):

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1.
2.
3.

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

Hello !

Indeed, the requirements have to be updated.<|||||>Has the problem been solved? I met the same issue when loading the transformers.<|||||>Hello, you have to have TF 2.3 at min. This will be fixed in the next release.<|||||>This breaks at least a couple of the tutorial notebooks. Even with TF 2.3.0 I get the same error.<|||||>If you get this message error:
```
AttributeError: module 'tensorflow_core.python.keras.api._v2.keras.activations' has no attribute 'swish'
```
It means you don't have at least TF 2.2 installed.<|||||>The problem can be seen as Transformers uses the tf _swish activation function_ by default (that does not exists in tf 2.1: https://www.tensorflow.org/versions/r2.1/api_docs/python/tf/keras/activations).

A workaround, instead of upgrading tf to 2.2 (unavailable at this time with `conda`), is to downgrade Transformers to a version that was developed with tf 2.1. 

For example, I had this warning with TF 2.1.0 and transformers 3.5.1 and it desappeared with transformers 3.0.2 [Warning : this version installs a specific version of pytorch so it is better to use a virtual environment].<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.