Support list of tensors in Metrics

🚀 Feature

Related to discussion from here: https://github.com/Project-MONAI/MONAI/issues/2220#issuecomment-849250700

The feature request is to support list of tensors or scalars as input for metrics: a) classification problem:

acc = Accuracy()
output = (
    [torch.rand(10), torch.rand(10), torch.rand(10)],  # predictions
    [4, 5, 1],  # ground truth
)
acc.update(output)
print(acc.compute())

b) segmentation problem:

acc = Accuracy()
output = (
    [torch.rand(10, 32, 32), torch.rand(10, 40, 30), torch.rand(10, 30, 24)],  # predictions
    [torch.randint(0, 10, size=(32, 32)), torch.randint(0, 10, size=(40, 30)),  torch.randint(0, 10, size=(30, 24))],  # ground truth
)
acc.update(output)
print(acc.compute())

c) Same for API with Engine.

cc @Nic-Ma

[vfdev-5]

I think once we are OK with details on what to add, we can try to implement it and schedule for this release.

[Nic-Ma]

Sounds great!

Thanks for your update.