Registrable _to_params default functionality

[gabeorlanski]

Fixes #5367 .

Changes proposed in this pull request:

• Added in a default behavior to the _to_params method of Registrable so that in the case it is not implemented by the child class, it will still produce a parameter dictionary.
• Added in _to_params implementations to all tokenizers.

The motivation for this PR is that currently, almost every Registrable does not have an implementation of _to_params and thus will raise a NotImplementedError when trying to populate a config with parameters. Thus, adding minimal functionality to the Registrable class avoids this error.

It is not the cleanest solution and implementing the _to_params method for all classes would be better, but also time-consuming.

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