Index Tree Fusion Does Not Work for MTTKRP

[johnpzh]

The current fusion implementation of the Index Tree dialect does not work for MTTKRP. MTTKRP's Index Tree dialect after fusion does not change and still has 2 roots (itree nodes).

The correct fused index tree should only have 1 root and 3 compute nodes.

// MTTKRP: 
Aij = Bikl * Clj * Dkj
// -> Current index tree has 2 roots and each one has 1 compute node
Tikj = Bikl * Clj
Aij = Tikj * Dkj
// -> Fused index tree should have 1 root with 3 compute nodes
Tj = 0
Tj = Bikl * Clj
Aij = Tj * Dkj

[gkestor]

We need to a cost model to enable fusion. The preliminary experiments demonstrate that fusion of dense MTTKRP is not efficient due to low data locality.