Update type of PrecisionPlugin.run_backward

[awaelchli]

What does this PR do?

Part of #9987

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

[tchaton]

LGMT !

[daniellepintz]

@awaelchli this PR is causing a type error in #10682:

pytorch_lightning/plugins/precision/deepspeed.py:52: error: Argument 2 of "_run_backward" is incompatible with supertype "PrecisionPlugin"; supertype defines the argument type as "Optional[Module]"  [override]
pytorch_lightning/plugins/precision/deepspeed.py:52: note: This violates the Liskov substitution principle
pytorch_lightning/plugins/precision/deepspeed.py:52: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides

pytorch_lightning/plugins/precision/native_amp.py:65: error: Argument 2 of "_run_backward" is incompatible with supertype "PrecisionPlugin"; supertype defines the argument type as "Optional[Module]"  [override]
pytorch_lightning/plugins/precision/native_amp.py:65: note: This violates the Liskov substitution principle
pytorch_lightning/plugins/precision/native_amp.py:65: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides

should I set the type to Optional[Module] for deepseed and native_amp as well?

[awaelchli]

Hmm, for native amp that would be ok, but in deepspeed it's not optional. I don't know how to express this with mypy.

[awaelchli]

I'll send a fix for this, it will be ugly.