correct the padding related calculation errors in SSIM

[petertheprocess]

What does this PR do?

Fixes #2718 ,
Before the convolution, we already applied same padding to the image

        preds = F.pad(preds, (pad_w, pad_w, pad_h, pad_h), mode="reflect")
        target = F.pad(target, (pad_w, pad_w, pad_h, pad_h), mode="reflect")

So ssim_idx_full_image will share the same shape as original image, there is no need to take the padding out again, otherwise it will result in losing information on edge.

    ssim_idx_full_image = ((2 * mu_pred_target + c1) * upper) / ((mu_pred_sq + mu_target_sq + c1) * lower)

    if is_3d:
        ssim_idx = ssim_idx_full_image[..., pad_h:-pad_h, pad_w:-pad_w, pad_d:-pad_d]
    else:
        ssim_idx = ssim_idx_full_image[..., pad_h:-pad_h, pad_w:-pad_w]

Comparison

 preds = torch.rand([3, 3, 256, 256])
 # let the edge of the image be 0
 target = preds.clone()
 target[:, :, 0, :] = 0
 target[:, :, -1, :] = 0
 target[:, :, :, 0] = 0
 target[:, :, :, -1] = 0
 print(structural_similarity_index_measure(preds, target))

Before this PR

tensor(1.0000), which should be wrong because the edge are different.

After this PR

tensor(0.9836), which make sense.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

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If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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---

📚 Documentation preview 📚: https://torchmetrics--2721.org.readthedocs.build/en/2721/

[Borda]

can we add test for this fix, pls

[petertheprocess]

Since we modified the calculation method, all previous quantitativ test will be affected.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 38%. Comparing base (45a41b8) to head (7f97f07).
Report is 96 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (45a41b8) and HEAD (7f97f07). Click for more details.

HEAD has 160 uploads less than BASE

Flag	BASE (45a41b8)	HEAD (7f97f07)
Windows	8	3
python3.8	9	3
cpu	59	20
torch1.13.1+cpu	9	3
macOS	12	4
python3.10	15	5
torch2.0.1	6	2
python3.11	8	3
torch2.4.0	3	1
torch2.4.0+cpu	2	1
torch1.11.0+cpu	3	1
Linux	39	13
python3.9	27	9
torch1.12.1+cpu	3	1
torch1.10.2+cpu	3	1
torch2.0.1+cpu	9	3
torch1.13.1	3	1
torch2.3.1+cpu	6	2
torch2.2.2+cpu	6	2
torch2.1.2+cpu	3	1
torch2.4.0+cu121	3	1
gpu	2	0
unittest	2	0

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2721     +/-   ##
========================================
- Coverage      69%     38%    -31%     
========================================
  Files         329     316     -13     
  Lines       18064   17900    -164     
========================================
- Hits        12492    6761   -5731     
- Misses       5572   11139   +5567