Default values for inputs?

[justinchuby]

Sometimes we would like to have default values for inputs (e.g. baddbmm alpha), or dynamic attributes.

We could make them optional, but support for static optional in functions isn’t mature yet when they are involved in logic.

I wonder if we can store the default when we parse the function definition and add it as a constant in the graph if the input isn’t provided by the caller.

cc @titaiwangms @BowenBao

[justinchuby]

Will support naturally with eager mode.