[PASS] Improve graph fusion (#286)
[PASS] Improve graph fusion (#286)

* [PASS] Improve graph fusion

* Change fusion center to segment head

* Use 'master' to identity the schedule node

* Make things compact

* Fix


Laurawly/tvm-1,Laurawly/tvm-1,Huyuwei/tvm,tqchen/tvm,tqchen/tvm,sxjscience/tvm,dmlc/tvm,Huyuwei/tvm,sxjscience/tvm,phisiart/tvm,Laurawly/tvm-1,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,tqchen/tvm,Laurawly/tvm-1,phisiart/tvm,sxjscience/tvm,Huyuwei/tvm,Huyuwei/tvm,phisiart/tvm,Huyuwei/tvm,sxjscience/tvm,phisiart/tvm,tqchen/tvm,Huyuwei/tvm,sxjscience/tvm,Laurawly/tvm-1,tqchen/tvm,dmlc/tvm,tqchen/tvm,phisiart/tvm,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,sxjscience/tvm,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,dmlc/tvm,sxjscience/tvm,dmlc/tvm,tqchen/tvm,tqchen/tvm,sxjscience/tvm,phisiart/tvm,Huyuwei/tvm,dmlc/tvm,sxjscience/tvm,phisiart/tvm,tqchen/tvm,Laurawly/tvm-1,Huyuwei/tvm,Huyuwei/tvm