Better handle types and overflows in softmax (especially lagacy and stable)

[jmitrevs]

Description

The legacy softmax was prone to overflow, so this PR made it use an accumulator type. Also, for stable, the default types are better chosen. There are some changes in latency, as well, to avoid overflow, but my feeling is that latency can often behave badly, so this doesn't fix that.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change which adds functionality)

Tests

The standard softmax tests should still pass. One could consider adding specific overflow tests, like we have in the current version of the brevitas tutorial.

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.