Conda dependencies conflict with pip dependencies
Author: ZOUG
Pull request: no

## 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: 3.5.1, 3.3.1
- Platform: Windows 10, Anaconda
- Python version: 3.8

## Information

I'm installing a package built on top of `transformers` v3 in an Anaconda environment. The package is not available on Anaconda Cloud so I have to install it via `pip`. According to [the best practice](https://www.anaconda.com/blog/using-pip-in-a-conda-environment), I try to install as many requirements as possible with `conda`, including `transformers`. However, it turns out that the conda dependencies conflict with pip dependencies for `transformers` so that pip would try to downgrade the conda-installed `tokenizers` package, which `transformers` depends on.

The dependency information is as follows:
<table style="width:100%">
  <tr>
    <th>`transformers` version</th>
    <th>conda dependency</th>
    <th>pip dependency</th>
  </tr>
  <tr>
    <td>3.5.1</td>
    <td>tokenizers==0.9.4</td>
    <td>tokenizers==0.9.3</td>
  </tr>
  <tr>
    <td>3.3.1</td>
    <td>tokenizers==0.9.3</td>
    <td>tokenizers==0.8.1rc2</td>
  </tr>
</table>

I can't seem to find any resolution other than leaving the `transformers` installation to pip completely. Is there any other possible resolution?

### Who can help
Maybe @mfuntowicz can help

## To reproduce

Steps to reproduce the behavior:

1. Run "conda install transformers=3.5.1" or "conda install transformers=3.3.1"
2. Run "pip check"

## Expected behavior

Make conda dependencies compatible with pip dependencies.

Comments:
Hey @ZOUG,

Thanks for the issue. @LysandreJik is on holiday at the moment, but I'm sure he's more than happy to take a look when he's back :-) 

Hello! We have started officially maintaining the anaconda packages in version v4.0.0. Installing a version anterior to that one would result in you using the `transformers` version from another channel (such as `conda-forge`), which we do not maintain.

Do you get the same error when installing `transformers` from our channel (on a more recent version)?

> Hello! We have started officially maintaining the anaconda packages in version v4.0.0. Installing a version anterior to that one would result in you using the `transformers` version from another channel (such as `conda-forge`), which we do not maintain.
> 
> Do you get the same error when installing `transformers` from our channel (on a more recent version)?

No, the error does not occur on the most recent version. The problem is that packages dependent on `transformers` may not be compatible with v4.x at the moment so that the error will still arise. It might be better to provide a pip installation package for v3.5.1 that is compatible with the `conda-forge` dependencies.

In my case, I got lucky that the package that I need just released a new version today that is compatible with transformers v4.x.

I believe this is still the case in Docker-based environments (ex. Kaggle). I removed existing transformers and tokenizers, installed new ones (transformers 4.2.1 and tokenizers 0.9.4). 
In the code, it goes back to conda and complains about tokenizers being 0.9.3
```
/opt/conda/lib/python3.7/site-packages/transformers/__init__.py in <module>
     41 
     42 # Check the dependencies satisfy the minimal versions required.
---> 43 from . import dependency_versions_check
     44 from .file_utils import (
     45     _BaseLazyModule,

/opt/conda/lib/python3.7/site-packages/transformers/dependency_versions_check.py in <module>
     39                 continue  # not required, check version only if installed
     40 
---> 41         require_version_core(deps[pkg])
     42     else:
     43         raise ValueError(f"can't find {pkg} in {deps.keys()}, check dependency_versions_table.py")

/opt/conda/lib/python3.7/site-packages/transformers/utils/versions.py in require_version_core(requirement)
     92     """ require_version wrapper which emits a core-specific hint on failure """
     93     hint = "Try: pip install transformers -U or pip install -e '.[dev]' if you're working with git master"
---> 94     return require_version(requirement, hint)
     95 
     96 

/opt/conda/lib/python3.7/site-packages/transformers/utils/versions.py in require_version(requirement, hint)
     85     if want_ver is not None and not ops[op](version.parse(got_ver), version.parse(want_ver)):
     86         raise pkg_resources.VersionConflict(
---> 87             f"{requirement} is required for a normal functioning of this module, but found {pkg}=={got_ver}.{hint}"
     88         )
     89 

VersionConflict: tokenizers==0.9.4 is required for a normal functioning of this module, but found tokenizers==0.9.3.
``` 
Edit: found a work around to re-import modules:
```
import importlib, pkg_resources, tokenizers
importlib.reload(pkg_resources)
importlib.reload(tokenizers)
``` 
tqdm may also complain if  4.50 or later.

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.

Issue still persists

---------------------------------------------------------------------------
VersionConflict                           Traceback (most recent call last)
<ipython-input-24-3b738e6ed358> in <module>
----> 1 from transformers import PreTrainedTokenizerFast

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/importlib/_bootstrap.py in _find_and_load(name, import_)

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/importlib/_bootstrap.py in _find_and_load_unlocked(name, import_)

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/importlib/_bootstrap.py in _load_unlocked(spec)

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/importlib/_bootstrap.py in _load_backward_compatible(spec)

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/site-packages/transformers-4.2.2-py3.8.egg/transformers/__init__.py in <module>
     41 
     42 # Check the dependencies satisfy the minimal versions required.
---> 43 from . import dependency_versions_check
     44 from .file_utils import (
     45     _BaseLazyModule,

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/importlib/_bootstrap.py in _find_and_load(name, import_)

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/importlib/_bootstrap.py in _find_and_load_unlocked(name, import_)

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/importlib/_bootstrap.py in _load_unlocked(spec)

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/importlib/_bootstrap.py in _load_backward_compatible(spec)

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/site-packages/transformers-4.2.2-py3.8.egg/transformers/dependency_versions_check.py in <module>
     39                 continue  # not required, check version only if installed
     40 
---> 41         require_version_core(deps[pkg])
     42     else:
     43         raise ValueError(f"can't find {pkg} in {deps.keys()}, check dependency_versions_table.py")

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/site-packages/transformers-4.2.2-py3.8.egg/transformers/utils/versions.py in require_version_core(requirement)
     92     """ require_version wrapper which emits a core-specific hint on failure """
     93     hint = "Try: pip install transformers -U or pip install -e '.[dev]' if you're working with git master"
---> 94     return require_version(requirement, hint)
     95 
     96 

/app1/anaconda3/envs/praveen_tfu/lib/python3.6/site-packages/transformers-4.2.2-py3.8.egg/transformers/utils/versions.py in require_version(requirement, hint)
     85     if want_ver is not None and not ops[op](version.parse(got_ver), version.parse(want_ver)):
     86         raise pkg_resources.VersionConflict(
---> 87             f"{requirement} is required for a normal functioning of this module, but found {pkg}=={got_ver}.{hint}"
     88         )
     89 

VersionConflict: tokenizers==0.9.4 is required for a normal functioning of this module, but found tokenizers==0.11.6.
Try: pip install transformers -U or pip install -e '.[dev]' if you're working with git master