Disable batch_size extraction for torchmetric instances

[rohitgr7]

What does this PR do?

We don't need to extract batch_size if it's a torchmetric instance. Adds a small performance optimization.

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Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @Borda @carmocca @edward-io @ananthsub

[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[tchaton]

LGTM !