Adds method chaining support to `LightningModule.freeze()` and `LightningModule.unfreeze()` by returning `self` (#21469)

* return self instance in `freeze` and `unfreeze` method

* docs: fix docs

* add chlog

* docs: update freeze method docstring to use code-block directive

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Co-authored-by: Bhimraj Yadav <bhimrajyadav977@gmail.com>
Co-authored-by: thomas chaton <thomas@grid.ai>