[Relay][Keras] force const dtype to be float32 (#2376)
[Relay][Keras] force const dtype to be float32 (#2376)

* [Relay][Keras] force const dtype to be float32

* fix pylint

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