move functional psnr & ssim to image

[Borda]

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What does this PR do?

Fixes # (issue).

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[Borda]

@SkafteNicki just see that PSNR is used also in info retrieval, so shall it really live in image?

[codecov]

Codecov Report

Merging #382 (7cf94e1) into master (a5af6d8) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #382      +/-   ##
==========================================
- Coverage   96.46%   96.44%   -0.02%     
==========================================
  Files         114      113       -1     
  Lines        3705     3690      -15     
==========================================
- Hits         3574     3559      -15     
  Misses        131      131              

Flag	Coverage Δ
Linux	76.53% <100.00%> (-0.10%)	⬇️
Windows	76.53% <100.00%> (-0.10%)	⬇️
cpu	96.39% <100.00%> (-0.02%)	⬇️
gpu	96.36% <100.00%> (-0.02%)	⬇️
macOS	96.39% <100.00%> (-0.02%)	⬇️
pytest	96.44% <100.00%> (-0.02%)	⬇️
python3.6	95.55% <100.00%> (-0.02%)	⬇️
python3.8	96.39% <100.00%> (-0.02%)	⬇️
python3.9	96.28% <100.00%> (-0.02%)	⬇️
torch1.3.1	95.55% <100.00%> (-0.02%)	⬇️
torch1.4.0	95.63% <100.00%> (-0.02%)	⬇️
torch1.9.0	96.28% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
torchmetrics/functional/image/psnr.py	96.96% <ø> (ø)
torchmetrics/functional/image/ssim.py	100.00% <ø> (ø)
torchmetrics/regression/__init__.py	100.00% <ø> (ø)
torchmetrics/__init__.py	100.00% <100.00%> (ø)
torchmetrics/functional/__init__.py	100.00% <100.00%> (ø)
torchmetrics/functional/image/__init__.py	100.00% <100.00%> (ø)
torchmetrics/functional/regression/__init__.py	100.00% <100.00%> (ø)
torchmetrics/image/psnr.py	100.00% <100.00%> (ø)
torchmetrics/image/ssim.py	100.00% <100.00%> (ø)

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