support custom dir

[zhewenl]

Summary

This PR adds an option to specify BC linter config path, which will be used in vllm-project/vllm#21234

Test plan

Locally test against PR: vllm-project/vllm#24614

Before:

../test-infra/tools/stronghold/bin/check-api-compatibility --base-commit=b5e383cd8b62975dec605bed05e22d273c296c7a --head-commit=38ce6fa81660f0be6e0e61672fdc499bcd7fabc4

::group::fetch github.event.pull_request.base.sha
From ssh://github.com/zhewenl/vllm
 * branch                b5e383cd8b62975dec605bed05e22d273c296c7a -> FETCH_HEAD
::endgroup::
fatal: path 'vllm/_bc_linter.py' exists on disk, but not in 'b5e383cd8b62975dec605bed05e22d273c296c7a'
::warning file=vllm/v1/core/sched/output.py,line=99::Function CachedRequestData: num_computed_tokens changed from list[int] to list[float]
::warning file=vllm/v1/core/sched/scheduler.py,line=1::Function Scheduler.get_grammar_bitmask: function deleted
::warning file=vllm/v1/core/sched/scheduler.py,line=1::Function Scheduler.update_draft_token_ids: function deleted

After:

../test-infra/tools/stronghold/bin/check-api-compatibility --base-commit=b5e383cd8b62975dec605bed05e22d273c296c7a --head-commit=38ce6fa81660f0be6e0e61672fdc499bcd7fabc4 --config-dir=.github

::group::fetch github.event.pull_request.base.sha
From ssh://github.com/zhewenl/vllm
 * branch                b5e383cd8b62975dec605bed05e22d273c296c7a -> FETCH_HEAD
::endgroup::
BC-linter: Using .bc-linter.yml (parsed successfully)
fatal: path 'vllm/_bc_linter.py' exists on disk, but not in 'b5e383cd8b62975dec605bed05e22d273c296c7a'
::warning file=vllm/v1/core/sched/output.py,line=99::Function CachedRequestData: num_computed_tokens changed from list[int] to list[float]
> ../test-infra/tools/stronghold/bin/check-api-compatibility --base-commit=b5e383cd8b62975dec605bed05e22d273c296c7a --head-commit=38ce6fa81660f0be6e0e61672fdc499bcd7fabc4

Note the difference in scheduler.py, before the we specifying the local config the bc linter yml was not respected - so the default rules are applied and we check ALL python files; after specifying the yml the warnings are restricted to ONLY the functions with annotations

[vercel]

@zhewenl is attempting to deploy a commit to the Meta Open Source Team on Vercel.

A member of the Team first needs to authorize it.

[izaitsevfb]

is there a reason to pass these as two separate params, instead of a single concatenated path to load config from?

[izaitsevfb]

same here

[izaitsevfb]

lgtm overall, but please check the question in the comments