Default benchmark based on deterministic flag

[krshrimali]

What does this PR do?

• This PR makes the benchmark flag of Trainer class Optional, and defaults to the value based on deterministic flag. Also allows manually overwriting the flag.
• Why not deprecate? We wish to allow overwriting the flag by the users, and prevent BC-breaking changes.
• Tests have been updated accordingly.

Fixes #9128.

Does your PR introduce any breaking changes? If yes, please list them.

No!

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Did you have fun?

Thanks to @carmocca for helping me with this PR. 🎉

[carmocca]

LGTM!

I pushed one commit with docs changes after searching for how to properly reference another parameter. Also added one reference to PyTorch's docs about randomness.

[kaushikb11]

LGTM!

[awaelchli]

GTLM

[rohitgr7]

LGTM
small comments.