[Bug Report] ActivationsStore fails for models without a tokenizer

[chanind]

Describe the bug
For models without a tokenizer, e.g. Othello, an error is thrown in the ActivationsStore since it assumes a tokenizer is present and attempts to insert BOS tokens. If the model has no Tokenizer, we should gracefully handle this and not attempt to read anything form the tokenizer.

Code example
See #177

Checklist

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