Refactor `tiktoken` import bare except

[ringohoffman]

We shouldn't suppress the import error if tiktoken is not installed. Also do the same thing for sentencepiece.

Instead of importing them at the top of the file, we can import them inside the only functions that use them; we can consider a different import structure if we end up needing to access them elsewhere.

This lazy importing should also decrease the load time of these modules.

cc: @msaroufim

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