make FX symbolic tracing reuse buffers if they're the same (#66211)
Summary:
Currently, if the same tensor constant is reused multiple times, we'll store a tensor constant for each time we use it.
For example
```
val = torch.randn(5)
for _ in range(10):
x = x + val
```
ends up storing 10 tensor constants.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66211
Reviewed By: jamesr66a
Differential Revision: D31437089
Pulled By: Chillee
fbshipit-source-id: 401169c8d58ce0afb7025ae11060680ef544419f
