[quant][graphmode] Handling ops doesn't require observation in insertObservers (#33481)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33481
We have to propagate observed property of values through ops like max_pool2d, flatten and
avoid inserting duplicated observers.
For example:
```
x1 = self.conv(x)
x2 = maxpool(x1)
x3 = self.conv(x2)
```
If x1 is observed, we should propagate this information through maxpool and
we should consider x2 as observed as well.
Test Plan:
python test/test_jit.py
Imported from OSS
Differential Revision: D20261897
fbshipit-source-id: 7de354a3ccb2b6e1708f5c743d4d9f7272691a93
