move datasets to existing package

[Borda]

What does this PR do?

There is already package datasets with just Dummy datasets, so lets move all datasets there...
TODO: think why we need to name all datasets in file name when it is already in folder name
import pl_bolts.datasets.mnist_dataset ?

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[codecov]

Codecov Report

Merging #275 into master will decrease coverage by 2.56%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #275      +/-   ##
==========================================
- Coverage   82.46%   79.90%   -2.57%     
==========================================
  Files          89       91       +2     
  Lines        4664     4841     +177     
==========================================
+ Hits         3846     3868      +22     
- Misses        818      973     +155     

Flag	Coverage Δ
#cpu	23.92% <25.00%> (-1.34%)	⬇️
#pytest	23.92% <25.00%> (-1.34%)	⬇️
#unittests	79.36% <100.00%> (-2.55%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pl_bolts/datasets/base_dataset.py	95.45% <ø> (ø)
pl_bolts/datasets/concat_dataset.py	41.66% <ø> (ø)
pl_bolts/datasets/imagenet_dataset.py	18.78% <ø> (ø)
pl_bolts/datasets/kitti_dataset.py	30.43% <ø> (ø)
pl_bolts/datasets/mnist_dataset.py	42.85% <ø> (ø)
pl_bolts/datasets/ssl_amdim_datasets.py	73.23% <ø> (ø)
pl_bolts/datamodules/__init__.py	100.00% <100.00%> (ø)
pl_bolts/datasets/cifar10_dataset.py	96.80% <100.00%> (ø)
pl_bolts/models/self_supervised/amdim/datasets.py	60.37% <100.00%> (ø)
...ts/models/autoencoders/basic_ae/basic_ae_module.py	79.72% <0.00%> (-8.11%)	⬇️
... and 4 more

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