[LT] Store OpKind for each IR subclass in a static field
Summary: Currently OpKind is stored as an object field called op_ for each IR
node, and one usage of op_ is to avoid dynamic_cast in NodeCast when we
need to downcast a base-node pointer into a concrete sub-node pointer.
As a result, we need to construct and pass in an op when downcasting
nodes, and this becomes quite anonnying when we start to implement the
trie-based IR node reusing. More importantly, the op for each subclass
should be unique for that subclass and thus making it a const static field
is a more logical design.
In this PR, we still keep the object-level op_ for easier XLA adoption. As
furture work, we can come back to remove op_, make the op() method
virtual, and get rid of OpKind in all the node constructors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76711
Approved by: https://github.com/wconstab, https://github.com/JackCaoG
