Fix a minor potential i32 overflow in thrust::transform_exclusive_scan in PQ reader preprocessing

[mhaseeb123]

Description

This PR fixes a minor potential int32 overflow in thrust::transform_exclusive_scan by setting the init parameter to the correct desired output type (size_t{0}). The counting iterator is also updated size_t type to match pass.chunks.size()'s type, though it's unlikely we will have >2B chunks per pass.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[mhaseeb123]

/merge