pl version: examples/requirements.txt is single source of truth

[stas00]

PL git master is unstable:

cd examples/text-classification
./run_pl.sh

   File "run_pl_glue.py", line 12, in <module>
    from lightning_base import BaseTransformer, add_generic_args, generic_train
  File "/mnt/nvme1/code/huggingface/transformers-master/examples/lightning_base.py", line 7, in <module>
    import pytorch_lightning as pl
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/__init__.py", line 76, in <module>
    __import__('pkg_resources').declare_namespace(__name__)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2301, in declare_namespace
    _handle_ns(packageName, path_item)
  File "/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pkg_resources/__init__.py", line 2234, in _handle_ns
    loader.load_module(packageName)
  File "/mnt/nvme1/code/github/00pytorch/pytorch-lightning/pytorch_lightning/__init__.py", line 56, in <module>
    from pytorch_lightning.core import LightningDataModule, LightningModule
ImportError: cannot import name 'LightningDataModule' from 'pytorch_lightning.core' (/home/stas/anaconda3/envs/main/lib/python3.7/site-packages/pytorch_lightning/core/__init__.py)

the scripts will now rely on:

grep pytorch-l examples/requirements.txt

pytorch-lightning==0.8.5

whenever the requirement removed by this PR was added (it also helps to add why it was added):

# Install newest ptl.
pip install -U git+http://github.com/PyTorchLightning/pytorch-lightning/

seems to no longer be needed - at least the code runs to completion.

[codecov]

Codecov Report

Merging #6309 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6309      +/-   ##
==========================================
+ Coverage   79.44%   79.52%   +0.07%     
==========================================
  Files         148      148              
  Lines       27193    27193              
==========================================
+ Hits        21604    21625      +21     
+ Misses       5589     5568      -21     

Impacted Files	Coverage Δ
src/transformers/modeling_tf_flaubert.py	24.53% <0.00%> (-63.20%)	⬇️
src/transformers/tokenization_bart.py	60.56% <0.00%> (-35.22%)	⬇️
src/transformers/generation_tf_utils.py	86.46% <0.00%> (+1.00%)	⬆️
src/transformers/tokenization_dpr.py	57.65% <0.00%> (+4.50%)	⬆️
src/transformers/modeling_tf_distilbert.py	97.41% <0.00%> (+32.94%)	⬆️

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[LysandreJik]

This LGTM, thanks @stas00.

@sshleifer, what do you think of this?

[sshleifer]

I am strongly in favor.