Fix loop type (#50484)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50484
I currently see the compilation warning:
```
Jan 13 16:46:21 [3644/5223] Building CXX object caffe2/CMakeFiles/torch_cpu.dir/__/aten/src/ATen/core/ivalue.cpp.o
Jan 13 16:46:21 ../aten/src/ATen/core/ivalue.cpp:855:22: warning: comparison of integers of different signs: 'int' and 'std::__1::vector<c10::IValue, std::__1::allocator<c10::IValue> >::size_type' (aka 'unsigned long') [-Wsign-compare]
Jan 13 16:46:21 for (auto i = 0; i < slots_.size(); ++i) {
```
This diff fixes that
Test Plan: Sandcastle tests
Reviewed By: ngimel
Differential Revision: D25901674
fbshipit-source-id: 0a09570866f23b5878bf06f46f918d71a733974f
