add default value for leaky relu alpha (#7259)
add default value for leaky relu alpha (#7259)


dmlc/tvm,dmlc/tvm,dmlc/tvm,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,dmlc/tvm,dmlc/tvm,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,dmlc/tvm,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1,Laurawly/tvm-1