[Relay][Hashing] Structural hash - incorporate the var type into its hash (#3267)

[Relay][Hashing] Structural hash - incorporate the var type into its hash (#3267)

Currently, the BindVar function does not take Var type into account. This causes
two same graph structures with different var shapes to have same hash.
Structural hash is used for keeping track of which operators we have
already compiled. Because of this, two operators with different shapes end up
pointing to same compiled code. The failure is encountered at runtime, where the
expected input shape asserts are not met.