Add custom_dummy_tokens to PretrainedTransformerTokenizer

[tarohi24]

This commit aims to support #5597

Changes proposed in this pull request:

• Add custom_dummy_tokens argument to PretrainedTransformerTokenizer. Without that argument, initializing PretrainedTransformerTokenizer of some non-English models fail.

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[epwalsh]

Thanks @tarohi24! This looks good, I just have two small suggestions

[tarohi24]

Thanks, @epwalsh for reviewing! The suggestions look good to me. I made commits to reflect these suggestions.

[epwalsh]

LGTM! Thanks @tarohi24!

[epwalsh]

Looks like we'll need to resolve #5613 before merging this.