[autockpt] linearize / merge shape-consistency nodes.

[super-dainiu]

Merge all shape-consistency nodes to a large node in CkptSolver.
This is because:

• shape-consistency nodes don't affect the performance of rematerialization
• shape-consistency nodes don't have intermediate activation
• reducing search space will speed up search time

[layer4_2_conv1, layer4_2_bn1, layer4_2_relu, layer4_2_conv2, layer4_2_bn2, layer4_2_relu_1, layer4_2_conv3, layer4_2_bn3, add_15, layer4_2_relu_2, runtime_comm_spec_apply] 0 9 0 9
[avgpool] 0 0 1 3
[flatten, fc_weight, fc_bias, runtime_apply_1, runtime_comm_spec_apply_1] 1 1 0 0
[linear, runtime_comm_spec_apply_2] 0 1 0 1