Design problems for Relay to support NLP models

[haichen]

@jroesch

I don’t think there is a reason to have attributes if they do not need to be static.

The only reason we introduced them is to separate the arguments list into static, and dynamic arguments.

We can still support keyword arguments, but this distinction is important as the attributes are not available at runtime, to me it complicates things to take a fragment of Relay program as an argument to statically code generate the operator.

After a second thought, I realize that for certain op like arange requires dynamic attributes. During the shape inference, it only has access to attributes. Therefore, when shape depends on dynamic value instead of input shapes, such as start/stop/step in arange, we have to put these dynamic values in the attributes.