Guard {set,rebase}_history on grad_fn check (#19623)
Summary:
We would previously have statements like
```
set_history(flatten_tensor_args( result ), grad_fn);
```
Internally, {set,rebase}_history would check grad_fn and short circuit if it is nullptr. However, this means that we are executing the expression `flatten_tensor_args( result )` and immediately throwing away the results. This was causing unnecessary allocations + overhead.
My JIT overhead benchmark script (with custom benchmark method):
```
import torch, time
torch.jit.script
def add(x, y):
return x + y
a = torch.rand([])
b = torch.rand([])
niter = 1000000
with torch.no_grad():
s = time.time()
add.__getattr__('forward').benchmark(niter, a, b)
e = time.time() - s
print('overhead per call (us)', e / niter * 1e6)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19623
Differential Revision: D15053399
Pulled By: jamesr66a
fbshipit-source-id: 8777e1a2b5c5a5bbd3a035b7247c8154c5fc4aa6
