fix: stream option is not working with tracing enabled

[nicoloboschi]

Fixes #2598

This is a regression introduced by the langsmith integration. It only happens when the model are configured with stream=True.

There are 2 issues:

1. The iterator is being consumed while converting data for langsmith so the end result is empty stream
2. there's an error cannot create 'generator' instances popping up in the logs during the conversion

[github-actions]

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_{Last Modified at 09 Jul 24 07:28 UTC}

[ogabrielluiz]

LGTM

[zzzming]

LGTM