Additional fix for PR#2972 (case of missing import)

[cbalint13]

It is an additional fix for #2972 as mentioned by @apivovarov .

@merrymercy ,

Can review confirm please ?

[apivovarov]

I'm not sure we want to import relay if user decided to use nnvm.compiler.build.
Can we use string comparison here? e.g.

if F.__name__ == 'tvm.relay.op':

[cbalint13]

I'm not sure we want to import relay if user decided to use nnvm.compiler.build.
Can we use string comparison here? e.g.

if F.__name__ == 'tvm.relay.op':

Did it. Ditto, in case of F == sym for consistency, idea is to remove import.

[cbalint13]

@apivovarov , @tqchen

Thank you all !