Support complex numbers in matmul.

[ematejska]

Imported from JIRA https://bugs.swift.org/browse/TF-629

Type:	Bug	Priority:	Minor
Reporter:	Anthony Platanios	Assignee:	Unassigned
Resolution:	Unresolved	Votes:	0
Labels:	None

Currently matmul invokes the batch matmul op whenever the input matrices have rank higher than 2. However, batch matmul takes adjoint flags as input instead of transpose flags. This means that when we support complex number types we should be passing conjugate(lhs) instead of lhs to the batch matmul op if transpose for lhs is true.