Changed max_steps default from None to -1

[EricWiener]

What does this PR do?

This PR addresses this comment. It changes max_steps to default to -1 and no longer allows None to be specified for max_steps.

Fixes #8818

The new combinations for max_epochs and max_steps are:
Only allow None for the max_epochs parameter and in the case (13), otherwise convert it to -1, this way we have:

	max_epochs	max_steps	Effect
1	-1	-1	Infinite training
2	-1	0	Do nothing
3	-1	>1	No epoch limits
4	0	-1	Do nothing
5	0	0	Do nothing
6	0	>1	Do nothing*
7	>1	-1	No step limits
8	>1	0	Do nothing
9	>1	>1	Stop when one hits
10	None	-1	Run for 1k epochs

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

Yes. You can no longer specify max_steps = None.

Not sure what needs to be done w/ regards to this.

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Did you have fun?

Always

[carmocca]

Looking great! Thanks for your work ❤️

[tchaton]

Great work ! Almost there.

[tchaton]

LGMT ! Great work !

[codecov]

Codecov Report

Merging #9460 (abf24d3) into master (76081fb) will increase coverage by 0%.
The diff coverage is 100%.

❗ Current head abf24d3 differs from pull request most recent head 8cb57ff. Consider uploading reports for the commit 8cb57ff to get more accurate results

@@          Coverage Diff           @@
##           master   #9460   +/-   ##
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  Coverage      89%     89%           
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  Files         182     182           
  Lines       16157   16153    -4     
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+ Hits        14322   14327    +5     
+ Misses       1835    1826    -9