Prevent sum overflow in broadcast_object_list (#70605)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70605
broadcast_object_list casted the sum of all object lengths to int from long causing overflows.
Test Plan:
Add a Tensor with >2GB storage requirement (in distributed_test.py) to object broadcast.
This Tensor is only added if test are running at Meta as github tests will oom.
Without fix the length will overflow and the program will request a negative sized Tensor:
```
RuntimeError: Trying to create tensor with negative dimension -2147482417: [-2147482417]
```
With fix it will pass the test.
Test used on server with GPUs:
buck test mode/dev-nosan //caffe2/test/distributed:distributed_nccl_spawn --local -- broadcast_object
buck test mode/dev-nosan //caffe2/test/distributed:distributed_gloo_spawn --local -- broadcast_object
Reviewed By: r-barnes
Differential Revision: D33405741
fbshipit-source-id: 972165f8297b3f5d475636e6127ed4a49adacab1
