Fix repeatedly invoke build_detection_model error.

[GuoxiaWang]

Bug

When I repeatedly invoke build_detection_model function to build the model, I will get the error: AttributeError: 'ResNet' object has no attribute 'layer1'.

To Reproduce

model = build_detection_model(cfg)
model = build_detection_model(cfg)

The reason

The variable ResNet50StagesTo4 is a global generator expression, so when we build the model secondly, the code executes to Line 82 for stage_spec in stage_specs: and stage_specs will return empty leading to do not add any stage. Finally, self._freeze_backbone try to freeze the backbone by executing m = getattr(self, "layer" + str(stage_index)) firstly. At the moment, it will throw the AttributeError AttributeError: 'ResNet' object has no attribute 'layer1'.

I guess you want to define ResNet50StagesTo4 as the tuple, so I try to fix by add tuple type qualifier.

The solution

Add the tuple type to ResNet50StagesTo5, ResNet50StagesTo4, ResNet50FPNStagesTo5, ResNet101FPNStagesTo5.

I do not know whether there are similar bugs existing, so you need to review my solution. Thank you!

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[fmassa]

LGTM, thanks a lot for finding and fixing this bug!

[ravichaurasia]

Thanks for the answer. Replacing resnet.py code to this worked for me

ResNet50StagesTo5 = tuple(
    StageSpec(index=i, block_count=c, return_features=r)
    for (i, c, r) in ((1, 3, False), (2, 4, False), (3, 6, False), (4, 3, True))
)
# ResNet-50 up to stage 4 (excludes stage 5)

ResNet50StagesTo4 = tuple(
    StageSpec(index=i, block_count=c, return_features=r)
    for (i, c, r) in ((1, 3, False), (2, 4, False), (3, 6, True))
)
# ResNet-50-FPN (including all stages)

ResNet50FPNStagesTo5 = tuple(
    StageSpec(index=i, block_count=c, return_features=r)
    for (i, c, r) in ((1, 3, True), (2, 4, True), (3, 6, True), (4, 3, True))
)
# ResNet-101-FPN (including all stages)

ResNet101FPNStagesTo5 = tuple(
    StageSpec(index=i, block_count=c, return_features=r)
    for (i, c, r) in ((1, 3, True), (2, 4, True), (3, 23, True), (4, 3, True))
)