Move image gradient

[SkafteNicki]

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Moves the image_gradient metric to a image folder (now that we have a image package).

PR review

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[Borda]

seems some type error appeared...

[codecov]

Codecov Report

Merging #381 (23e3ab3) into master (d6cb082) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
- Coverage   96.44%   96.36%   -0.09%     
==========================================
  Files         113      113              
  Lines        3690     3691       +1     
==========================================
- Hits         3559     3557       -2     
- Misses        131      134       +3     

Flag	Coverage Δ
Linux	76.53% <100.00%> (+<0.01%)	⬆️
Windows	76.53% <100.00%> (+<0.01%)	⬆️
cpu	76.53% <100.00%> (-19.86%)	⬇️
gpu	96.36% <100.00%> (+<0.01%)	⬆️
macOS	76.53% <100.00%> (-19.86%)	⬇️
pytest	96.36% <100.00%> (-0.09%)	⬇️
python3.6	?
python3.8	?
python3.9	?
torch1.3.1	?
torch1.4.0	?
torch1.9.0	?

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

Impacted Files	Coverage Δ
torchmetrics/functional/image/gradients.py	100.00% <ø> (ø)
torchmetrics/functional/__init__.py	100.00% <100.00%> (ø)
torchmetrics/functional/image/__init__.py	100.00% <100.00%> (ø)
torchmetrics/average.py	84.61% <0.00%> (-11.54%)	⬇️

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[Borda]

I had to remove the original image_gradients.py because it collided with the same-named function imported in __init__.py

UNEXPECTED EXCEPTION: TypeError("'module' object is not callable")
Traceback (most recent call last):
  File "/usr/lib/python3.8/doctest.py", line 1336, in __run
    exec(compile(example.source, filename, "single",
  File "<doctest torchmetrics.functional.image.gradients.image_gradients[3]>", line 1, in <module>
TypeError: 'module' object is not callable