Move vision models to pl_bolts.models.vision

[akihironitta]

What does this PR do?

Part of #427 and fixes #559.

# This will be disabled after this change:
from pl_bolts.models import GPT2, ImageGPT, SemSegment, UNet

# Use below after the change:
from pl_bolts.models.vision import GPT2, ImageGPT, SemSegment, UNet

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[codecov]

Codecov Report

Merging #561 (79a3104) into master (86e3f52) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #561   +/-   ##
=======================================
  Coverage   77.58%   77.58%           
=======================================
  Files         115      115           
  Lines        6701     6701           
=======================================
  Hits         5199     5199           
  Misses       1502     1502           

Flag	Coverage Δ
cpu	26.23% <100.00%> (ø)
pytest	26.23% <100.00%> (ø)
unittests	77.06% <100.00%> (ø)

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

Impacted Files	Coverage Δ
pl_bolts/models/vision/image_gpt/gpt2.py	100.00% <ø> (ø)
pl_bolts/models/__init__.py	100.00% <100.00%> (ø)
pl_bolts/models/vision/__init__.py	100.00% <100.00%> (ø)

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