[Relay] Add TopPattern to nn.dropout

[comaniac]

nn.dropout is usually simplified in inference, but not for training, so we need to make up this attribute to make sure the model with nn.dropout can pass all Relay passes.

cc @anijain2305

[Lyken17]

Was this related with https://discuss.tvm.apache.org/t/toppattern-has-not-been-registered-for-nn-dropout/11305?

@AndrewZhaoLuo mentioned that

Dropout isn’t really supported

But actually here is an registration.

[AndrewZhaoLuo]

Hey Lyken, this isn't really registering compute, instead it's registering an attribute saying this operator should not be fused with anything if I understand it correctly. Don't know the full context though.