support custom class in torchgen schema parser (#82925)
Differential Revision: [D38480514](https://our.internmc.facebook.com/intern/diff/D38480514/)
torchgen schema parser does not support parsing function schemas using custom class so far. Here is an example:
```
quantized::conv2d_relu.new(Tensor qx, __torch__.torch.classes.quantized.Conv2dPackedParamsBase packed_weight, float output_scale, int output_zero_point) -> (Tensor)
```
This PR parse custom class name and encapsulate that into an object of CustomClassType. The only thing we need right now is just store the string class name and return that in `__str__` method.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82925
Approved by: https://github.com/ezyang, https://github.com/bdhirsh
