Fix: Handle empty prompt arrays in make_seq2seq_fields (#483)

[Ashutosh0x]

Resolves #483. This PR fixes a crash in make_seq2seq_fields when an empty prompt array is provided. The previous logic attempted to create a target mask with negative dimensions (len(prompt) - 1) when prompt was empty. The fix adds conditional logic to generate a correct mask (based on response length) if the prompt is empty.

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[Ashutosh0x]

Hi @epot, just submitted this fix for the crash in make_seq2seq_fields when handling empty prompts (Issue #483). The logic now correctly handles the mask generation when prompt length is 0, preventing the negative dimension error in
p.zeros. Thanks for reviewing!

[Ashutosh0x]

I have signed the CLA.

[epot]

@Ashutosh0x I am not sure why you tagged me, but it's likely a mistake :)