Catch case of negative tokens to generate

[tdene]

What does this PR do ?

The current inference code that assigns a sane num_tokens_to_generate value should it not be specified, can in some cases lead to num_tokens_to_generate < 0. Having a negative value for num_tokens_to_generate breaks subsequent checks, and the engine crashes with tensor indexing errors.

Code that can assign a negative value to num_tokens_to_generate:

        if request.sampling_params.num_tokens_total is not None:
            request.sampling_params.num_tokens_to_generate = (
                request.sampling_params.num_tokens_total - len(request.prompt_tokens)
            )
        if request.sampling_params.num_tokens_to_generate is None:
            request.sampling_params.num_tokens_to_generate = self.context.max_sequence_length - len(
                request.prompt_tokens
            )

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