Ease constrain for fuse path in trt lower (#68148)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68148
Question raised regarding whether we should fuse pass a->b->c if node a has other consumer rather than node b. This diff is to ease the constrain in fuse path so that in case:
```
a
| |
b d
|
c
```
we still allow fuse path(a->b->c), after fuse, node b will be eliminated by dead_node_eliminator while node a keep in graph.
Reviewed By: yinghai, 842974287
Differential Revision: D32296266
fbshipit-source-id: 44ded07a97b5b708bdf37193a022fae21410b4bd
