Use compute_numel_overflow() in tensor_ptr builders

[lucylq]

Stack from ghstack (oldest at bottom):

• -> Use compute_numel_overflow() in tensor_ptr builders #19076
• Use safe_numel() in et #19075
• Add safe_numel() #19074

make_tensor_ptr() and related extension builders return TensorPtr (a shared_ptr) and cannot propagate Error, so the existing compute_numel()elementSize() size checks silently wrapped on overflow and let an undersized buffer back an oversized tensor (heap OOB). Migrate these callers to compute_numel_overflow() so overflow aborts, and add explicit c10::mul_overflows() guards around the subsequent numelelementSize multiplication. Touched: make_tensor_ptr(vector<uint8_t>), clone_tensor_ptr, empty_strided, and the two tensor_ptr.h template overloads.

Authored with Claude.

Differential Revision: D102082922

[pytorch-bot]

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