Adding FP8 weight export

[costigt-dev]

Reason for this PR

To enable export of OCP FP8 weights to onnx.

Changes Made in this PR

Float specific export mixins and handlers created, changes made to retrieve exponent and mantissa bit widths etc.

Testing Summary

Tests still being written

Risk Highlight

• This PR includes code from another work (please detail).
• This PR contains API-breaking changes.
• This PR depends on work in another PR (please provide links/details).
• This PR introduces new dependencies (please detail).
• There are coverage gaps not covered by tests.
• Documentation updates required in subsequent PR.

Makes changes to FloatQuant and the proxy class of FloatWeightBase which might be used elsewhere

Uses OCP fp8 types in pytorch which only work with version 2.1 onwards so this functionality won't work for some pytorch versions still within our test suite.

Relies on: #919 and is using placeholder code at present.

Checklist

• Code comments added to any hard-to-understand areas, if applicable.
• Changes generate no new warnings.
• Updated any relevant tests, if applicable.
• No conflicts with destination dev branch.
• I reviewed my own code changes.
• Initial CI/CD passing.
• 1+ reviews given, and any review issues addressed and approved.
• Post-review full CI/CD passing.

Future Work

[nickfraser]

See comments.