[vt] fix for crash when creating VtArray from large python buffer

[ablev-nv]

Description of Change(s)

The third parameter of std::accumulate will determine the return type for std::accumulate. If the type is not explicit, then the compiler will infer it. More info here:

https://en.cppreference.com/w/cpp/algorithm/accumulate

The Vt_ArrayFromBuffer helper uses std::accumulate to determine the number of items to be created in the VtArray. Because the third parameter given here is just 1, the compiler assumes this is an int and uses that for the return type from std::accumulate. This is fine for most sane uses of VtArray that are initialized from a Python buffer with less than 2 billion-ish items. But if your Python buffer has more than maxint items, then the int return type overflows, and the process usually crashes.

This change makes the third parameter explicitly Py_ssize_t, which is the type of the other inputs to std::accumulate and was likely the intended output type all along.

Fixes Issue(s)

Crash or incorrect behavior when creating a VtArray from a python buffer with more than maxint elements.

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[gitamohr]

Nice find and fix, thanks Alan!

[tallytalwar]

Filed as internal issue #USD-7719