Support variable batch size in throughput callback

[alex-hh]

What does this PR do?

This PR allows the throughput monitor callback to support variable batch size
Fixes #20235

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📚 Documentation preview 📚: https://pytorch-lightning--20236.org.readthedocs.build/en/20236/

[lantiga]

hey @alex-hh this is still in draft, are you planning to finish this up? I'll be happy to merge if you add a test