Improve type checker performance
Author: bschnurr
Pull request: yes

# What does this PR do?

conditionally declare `TOKENIZER_MAPPING_NAMES` within a `if TYPE_CHECKING` block so that type checkers don't need to evaluate the RHS of the assignment.

this improves performance of the pylance/pyright type checkers


```Python
from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("gpt2")

```

from 12 seconds, down to 2.5 seconds

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Fixes # (issue)


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Comments:
Last failure is not linked to this PR and has been fixed on master already, so we're good to go, thanks again!