Make transformers-tests job conditional on files changed

[dbarbuzzi]

SUMMARY:
Make transformers-tests job conditional on files changed by way of using the tj-actions/changed-files@v45 action to check which files changed (ignoring files/folders based on a separate trial: #1194).

TEST PLAN:
Verify this PR:

1. Correctly results in skipping the transformers-tests job
2. Allows merging with the skipped job

[github-actions]

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[dbarbuzzi]

@dsikka If this PR's current state works as expected, before merging, I want to make one change to how files are matched (change below). The change here is to include the test-check-transformers.yaml workflow file specifically, as any changes to this workflow should result in it being executed. As with other testing for this PR, I have tested the below changes in an external repo. It seems using the ! prefix does not work the same way with files_ignored which demands the switch to the files input approach instead.

         uses: tj-actions/changed-files@v45
         with:
-          files_ignore: |
-            examples/**
-            tests/e2e/**
-            tests/lmeval/**
-            tests/examples/**
-            **/*.md
-            .github/workflows/*
+          files: |
+            **
+            !examples/**
+            !tests/e2e/**
+            !tests/lmeval/**
+            !tests/examples/**
+            !**/*.md
+            !.github/**
+            .github/workflows/test-check-transformers.yaml