Allow `training_step` in manual optimization to return general mappings (#21011)

* change dict to mapping

* add a bit of testing

* changelog

* use custom mapping

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* Update tests/tests_pytorch/trainer/optimization/test_manual_optimization.py

Co-authored-by: Bhimraj Yadav <[email protected]>

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* update

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Bhimraj Yadav <[email protected]>
Co-authored-by: Deependu Jha <[email protected]>