Avoid string copies during Substr broadcasting.
Avoid string copies during Substr broadcasting.

Substr op implements broadcasting by allocating temporary tensors of the match
shapes. When the input string tensor contains large string tensors, this
procedure may blow up memory easily.

PiperOrigin-RevId: 357652876
Change-Id: I58723a2e8d653a72b0cfe32d0093c8da69cb94da

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