Prevent sum overflow in broadcast_object_list (#70336)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70336
broadcast_object_list casted the sum of all object lengths to int from long causing overflows.
Test Plan:
Increased size of Tensor used in object transfers to have >2GB storage requirement (in distributed_test.py)
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
Differential Revision: D33281300
fbshipit-source-id: 1bc83e8624edc14e747eeced7bc8a7a10e443ee4
