Fixes #5668 .

Description

The SSIM loss wasn't feeding into autograd - it wasn't allowing gradient to be calculated. Now they can be calculated.
As per my previous PR (#5550), the loss function is now calculated using the SSIMMetric (and not the other way around). The SSIMMetric in inheriting from class IterationMetric that runs .detach() on all inputs before running ._compute_tensor().

To Reproduce

import torch
from monai.losses.ssim_loss import SSIMLoss

x = torch.ones([1,1,10,10])/2
y = torch.ones([1,1,10,10])/2
y.requires_grad_(True)
data_range = x.max().unsqueeze(0)
loss = SSIMLoss(spatial_dims=2)(x,y,data_range)
print(loss.requires_grad)

Expected behavior
loss.requires_grad will now be True.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).