Fixes for Modular Converter on Windows

[hlky]

What does this PR do?

This PR fixes some issues with Modular Converter on Windows.

1. Relative path regex. rf"(src{os.sep}transformers{os.sep}.*|examples{os.sep}.*)" results in (src\transformers\.*|examples\.*), due to \t and \. this regex fails, we use \\ here instead.
2. Relative path in auto generated message. On Windows this generates as This file was automatically generated from src\transformers\models\florence2\modular_florence2.py. This could potentially cause issues elsewhere, and it's better to be standardized, so when os.sep == \ we replace os.sep with /.
3. open encoding. On Windows the default encoding is cp1252 which doesn't work with 🚨, so we use encoding=utf-8 instead for all open. This particular fix is in effect in many other places in the codebase.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@ArthurZucker

[hlky]

Please note the test failure here is unrelated, looks like modeling_glm needs to be overwritten with modular generated code.