Add possibility to ignore imports in test_fecther
# What does this PR do?

This PR adds the ability to ignore some imports in the internal mechanism of the tests fetcher. The issue is that a recent PR introduced a dependency on the auto modules for `tokenization_utils_base`, which is a module that often ends up in a dep. Since this is just to grab the tokenizer class and issue an error, the auto module (and then all the models) does not need to be tested if there is a change of tokenization_utils_base.

This is a short-term fix, but a longer term patch is to reduce the the cross-dependencies inside the library.
