[quantize] fix bug of annotate for output of add op

[MingkangW]

The type of add ops output is activation, it should annotate by QAnnotateKind.ACTIVATION. If not, the graph will cast int32 into int8 directly without quantized, when quantize resnet. It will product overflow error, and it wont prompt when infer the model. The resnet18_v1 test case is below.
before fixed
%703 = add(%701, %702) /* ty=Tensor[(1, 512, 7, 7), int32] /;
%704 = nn.relu(%703) / ty=Tensor[(1, 512, 7, 7), int32] /;
%705 = cast(%704, dtype="int8") / ty=Tensor[(1, 512, 7, 7), int8] /;
%706 = annotation.stop_fusion(%705) / ty=Tensor[(1, 512, 7, 7), int8] /;
after fixed
%443 = add(%441, %442) / ty=Tensor[(1, 512, 7, 7), int32] /;
%444 = nn.relu(%443) / ty=Tensor[(1, 512, 7, 7), int32] /;
%445 = cast(%444, dtype="int64") / ty=Tensor[(1, 512, 7, 7), int64] /;
%446 = fixed_point_multiply(%445, multiplier=1439683968, shift=-2) / ty=Tensor[(1, 512, 7, 7), int64] /;
%447 = clip(%446, a_min=-127f, a_max=127f) / ty=Tensor[(1, 512, 7, 7), int64] /;
%448 = cast(%447, dtype="int32") / ty=Tensor[(1, 512, 7, 7), int32] /;
%449 = cast(%448, dtype="int8") / ty=Tensor[(1, 512, 7, 7), int8] /;
%450 = annotation.stop_fusion(%449) / ty=Tensor[(1, 512, 7, 7), int8] */;

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[cbalint13]

@MingkangW ,
Can also add a small test case ?

[MingkangW]

@MingkangW ,
Can also add a small test case ?

Sure, I will do it

[MingkangW]

@cbalint13
I have added test cases. If the annotation for add is not modified to ACTIVATION, the test case will not pass.

[cbalint13]

@MingkangW ,
LGTM, thank you !

[MingkangW]

PTAL @cbalint13 @tqchen

[MingkangW]

cc: @masahi