update pytorch-lightning requirement to >= 1.8.0

[llimllib]

pytorch_lightning removed this variable in commit Lightning-AI/pytorch-lightning@7ef8746 part of Lightning-AI/pytorch-lightning#14550, which seems to have gone out in lightning 1.8.0.

• replace _TPU_AVAILABLE with TPUAccelerator.is_available, which seems to be what the above referenced commit uses in its place
• fix updated callback name on_batch_end -> on_train_batch_end
• update pytorch-lightning req in requirements.txt

Without the change in place, I get:

$ python -c "from aitextgen.train import ATGTransformer"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/llimllib/.venv/lib/python3.10/site-packages/aitextgen/__init__.py", line 1, in <module>
    from .aitextgen import aitextgen  # noqa
  File "/Users/llimllib/.venv/lib/python3.10/site-packages/aitextgen/aitextgen.py", line 31, in <module>
    from .train import ATGProgressBar, ATGTransformer
  File "/Users/llimllib/.venv/lib/python3.10/site-packages/aitextgen/train.py", line 14, in <module>
    from pytorch_lightning.utilities import _TPU_AVAILABLE
ImportError: cannot import name '_TPU_AVAILABLE' from 'pytorch_lightning.utilities' (/Users/llimllib/.venv/lib/python3.10/site-packages/pytorch_lightning/utilities/__init__.py)

With it in place, I get no output as expected

python -c "from aitextgen.train import ATGTransformer"

I was also able to use the example colab notebook with this branch, which I was not able to using the published version of aitextgen

fixes #200

[carmocca]

PyTorch Lightning dev here. LGTM!

[Morpheus636]

Maintainers, please merge this.