Add validate logic for precision

[SeanNaren]

What does this PR do?

Ensures that we validate the precision type when setting the precision plugin. Followup from #9049

Trainer(precision="x")

pytorch_lightning.utilities.exceptions.MisconfigurationException: Precision "x" is invalid. Allowed precision values: (16, 32, 64, 'bf16')

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[codecov]

Codecov Report

Merging #9080 (ddc2d2c) into master (f959b13) will increase coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9080    +/-   ##
=======================================
+ Coverage      88%     92%    +4%     
=======================================
  Files         176     176            
  Lines       14649   14671    +22     
=======================================
+ Hits        12875   13504   +629     
+ Misses       1774    1167   -607     

[kaushikb11]

LGTM!

[tchaton]

Looks great !

[awaelchli]

LGTM

[awaelchli]

Suggested change

	- Added validate logic for precision ([#9080](https://github.com/PyTorchLightning/pytorch-lightning/pull/9080))
	- Added input validation for precision Trainer argument ([#9080](https://github.com/PyTorchLightning/pytorch-lightning/pull/9080))

this should be less ambiguous