Fix PipelineTests skip conditions
# What does this PR do?

While I am starting to try to fix some pipeline tests that are skipped previously, I realized PR #21516 (removing the usage of meta class in pipeline testing) **accidentally skip more tests than it should**.

- previously, each combination of model/tokenizer/processor class has its own test case (being generated on the fly), so we can use `self.skipTest` to skip some cases.
- After #21516，each model + task has its test cases, but inside that test, it runs against all tokenizers/processors that are available. In particular, slow / fast tokenizer. 
  - As mentioned before, we have some slow tokenizer issues in pipeline testing (never tested before #20426), and we skip some failing cases for now.
  - but after #21516, when we use `self.skipTest`, we **might** **skip the next combination(s)**. This is **NOT good/expected**.

This PR instead uses `logger.warning` to log the skipped cases and continue to the next combination in a test. This is not very pleasant, but as we don't want to use meta class, this is the only way I can think of for now.

It's better to prepare a report file that clearly indicate which test cases and/or combination being [skipped].
_The documentation is not available anymore as the PR was closed or merged._