Fix IndexKernel.cu build (#104423)
Fixes `signed-unsigned comparison` warnings introduced by https://github.com/pytorch/pytorch/pull/106809 (previously by <s> https://github.com/pytorch/pytorch/pull/104054 </s> ) that changed type of `num_indices` to unsigned.
Before the change warnings looks as follows:
```
/tmp/tmpxft_00194ca7_00000000-6_IndexKernel.cudafe1.stub.c:31:580: required from here
/home/nshulga/git/pytorch/pytorch/aten/src/ATen/native/cuda/IndexKernel.cu:58:63: warning: comparison of integer expressions of different signedness: ‘const long unsigned int’ and ‘int’ [-Wsign-compare]
58 | AT_ASSERT(num_indices == iter.ntensors() - 2);
| ^
/home/nshulga/git/pytorch/pytorch/aten/src/ATen/native/cuda/IndexKernel.cu:74:19: warning: comparison of integer expressions of different signedness: ‘int’ and ‘const long unsigned int’ [-Wsign-compare]
74 | for (int i = 0; i < num_indices; i++) {
| ~~^~~~~~~~~~~~~
```
TODO: Turn those warning into errors
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104423
Approved by: https://github.com/Skylion007
