Describe the bug
SSIMLoss result does not have gradient
@PedroFerreiradaCosta
@wyli

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 be False. But it should be True

Screenshots
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Environment

Ensuring you use the relevant python executable, please paste the output of:

python -c 'import monai; monai.config.print_debug_info()'

Additional context
Add any other context about the problem here.