[Bugfix] Sets is_first_step_output for TPUModelRunner

[allenwang28]

#8378 introduced is_first_step_output - intended to be an optional field, but is not set as such. This broke TPU's multi-step runner. This PR fixes this by setting this variable in a similar way to multi_step_runner.

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[WoosukKwon]

LGTM. Thanks for the fix!