Allow training_step in manual optimization to return general mappings

[SkafteNicki]

What does this PR do?

Fixes #20993
Allows users doing manual optimization to return general mappings, including TensorDict as requested in the issue.
This also makes the supported output types consistent between manual and automatic optimization:

pytorch-lightning/src/lightning/pytorch/loops/optimization/automatic.py

Line 65 in d29839b

if isinstance(training_step_output, Mapping):

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📚 Documentation preview 📚: https://pytorch-lightning--21011.org.readthedocs.build/en/21011/