Version bump to 0.7.5

[snarayan21]

Description of changes:

bumping version to 0.7.5

Issue #, if available:

Merge Checklist:

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General

• I have read the contributor guidelines
• This is a documentation change or typo fix. If so, skip the rest of this checklist.
• I certify that the changes I am introducing will be backward compatible, and I have discussed concerns about this, if any, with the MosaicML team.
• I have updated any necessary documentation, including README and API docs (if appropriate).

Tests

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• I have added tests that prove my fix is effective or that my feature works (if appropriate).
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