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()`.

https://github.com/Project-MONAI/MONAI/blob/0737a33d62ce1e18023712a000828235b7758536/monai/metrics/metric.py#L70
I'm now calling the `SSIMMetric()._compute_tensor()` explicitly,
removing the step where it detaches from the current graph.

**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
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).

Signed-off-by: PedroFerreiradaCosta <[email protected]>