[ONNX] reduce log spam when exporting dropout in training mode (#78309)
The default for `torch.onnx.export` is `TrainingMode.EVAL`:
https://github.com/pytorch/pytorch/blob/0d76299ff782224810b997d99f8af490cd96eede/torch/onnx/__init__.py#L63
That means that this warning is only printed when the caller overrides
that and explicitly specifies that they want training ops like Dropout.
We should assume the user knows what they're doing and not warn.
Also set `do_constant_folding=False` in the dropout related training tests. Without this, warnings are printed like:
```
UserWarning: It is recommended that constant folding be turned off ('do_constant_folding=False') when exporting the model in training-amenable mode
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78309
Approved by: https://github.com/justinchuby, https://github.com/BowenBao
