fix invalid escape sequences in grouped_data.py

[bluenote10]

Why are these changes needed?

This PR fixes the invalid escape sequences in grouped_data.py reported in #48921

Related issue number

closes #48921, but only partially. It looks like there are many more invalid escape sequences in this code base. This means that the same issue can just re-occur in a different file.

The root cause is that the linting rule W605 is currently globally suppressed:

ray/.flake8

Line 29 in 19818cf

W605

A true fix of #48921 would require to enable this lint, and fix them all. If desired, I could look into that in a follow-up.

EDIT: After a closer inspection (python/mypy#18215 (comment)) it looks like the error is actually not related to invalid escape sequences, but due to non standard handling of lazy imports. I will address these issues separately in a follow-up.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[bluenote10]

After a closer inspection (python/mypy#18215 (comment)) it looks like the error is actually not related to invalid escape sequences, but due to non standard handling of lazy imports. I will address these issues separately in a follow-up.