Add MNIST dataset & drop torchvision dep. from tests (#986)

* added custom mnist without torchvision dep

* move files so it does not conflict with mnist gitignore

* mock torchvision for tests

* fix line too long

* fix line too long

* fix "module level import not at top of file" warning

* move mock imports to __init__.py

* simplify MNIST a lot and download directly the .pt files

* further simplify and clean up mnist

* revert import overrides

* make as before

* drop  PIL requirement

* move mnist.py to datasets subfolder

* use logging instead of print

* choose same name as in torchvision

* remove torchvision and pillow also from yml file

* refactor if train

Co-Authored-By: Jirka Borovec <Borda@users.noreply.github.com>

* capitalized class attr

* moved mnist to models

* re-added datsets ignore

* better name for file variable

* Update mnist.py

* move dataset classes to datasets.py

* new line

* update

* update

* fix automerge

* move to base folder

* adapt testingmnist to new mnist base class

* remove temporal fix

* fix datatype

* remove old testingmnist

* readable

* fix import

* fix whitespace

* docstring

Co-Authored-By: Jirka Borovec <Borda@users.noreply.github.com>

* Update tests/base/datasets.py

Co-Authored-By: Jirka Borovec <Borda@users.noreply.github.com>

* changelog

* added types

* Update CHANGELOG.md

Co-Authored-By: Jirka Borovec <Borda@users.noreply.github.com>

* exist->isfile

Co-Authored-By: Jirka Borovec <Borda@users.noreply.github.com>

* index -> idx

* temporary fix for trains error

* better changelog message

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

tests/base/debug.py

@@ -1,9 +1,9 @@
 import torch
 from torch.nn import functional as F
 from torch.utils.data import DataLoader
-from torchvision.datasets import MNIST
 
 import pytorch_lightning as pl
+from tests.base.datasets import MNIST
 
 
 # from test_models import assert_ok_test_acc, load_model, \