def build_trident_resnet_backbone(cfg, input_shape):
    """
    Create a ResNet instance from config for TridentNet.

    Returns:
        ResNet: a :class:`ResNet` instance.
    """
    # need registration of new blocks/stems?
    norm = cfg.MODEL.RESNETS.NORM
    stem = BasicStem(
        in_channels=input_shape.channels,
        out_channels=cfg.MODEL.RESNETS.STEM_OUT_CHANNELS,
        norm=norm,
    )
    freeze_at = cfg.MODEL.BACKBONE.FREEZE_AT

    if freeze_at >= 1:
        for p in stem.parameters():
            p.requires_grad = False
        stem = FrozenBatchNorm2d.convert_frozen_batchnorm(stem)

    # fmt: off
    out_features         = cfg.MODEL.RESNETS.OUT_FEATURES
    depth                = cfg.MODEL.RESNETS.DEPTH
    num_groups           = cfg.MODEL.RESNETS.NUM_GROUPS
    width_per_group      = cfg.MODEL.RESNETS.WIDTH_PER_GROUP
    bottleneck_channels  = num_groups * width_per_group
    in_channels          = cfg.MODEL.RESNETS.STEM_OUT_CHANNELS
    out_channels         = cfg.MODEL.RESNETS.RES2_OUT_CHANNELS
    stride_in_1x1        = cfg.MODEL.RESNETS.STRIDE_IN_1X1
    res5_dilation        = cfg.MODEL.RESNETS.RES5_DILATION
    deform_on_per_stage  = cfg.MODEL.RESNETS.DEFORM_ON_PER_STAGE
    deform_modulated     = cfg.MODEL.RESNETS.DEFORM_MODULATED
    deform_num_groups    = cfg.MODEL.RESNETS.DEFORM_NUM_GROUPS
    num_branch           = cfg.MODEL.TRIDENT.NUM_BRANCH
    branch_dilations     = cfg.MODEL.TRIDENT.BRANCH_DILATIONS
    trident_stage        = cfg.MODEL.TRIDENT.TRIDENT_STAGE
    test_branch_idx      = cfg.MODEL.TRIDENT.TEST_BRANCH_IDX
    # fmt: on
    assert res5_dilation in {1, 2}, "res5_dilation cannot be {}.".format(res5_dilation)

    num_blocks_per_stage = {50: [3, 4, 6, 3], 101: [3, 4, 23, 3], 152: [3, 8, 36, 3]}[depth]

    stages = []

    res_stage_idx = {"res2": 2, "res3": 3, "res4": 4, "res5": 5}
    out_stage_idx = [res_stage_idx[f] for f in out_features]
    trident_stage_idx = res_stage_idx[trident_stage]
    max_stage_idx = max(out_stage_idx)
    for idx, stage_idx in enumerate(range(2, max_stage_idx + 1)):
        dilation = res5_dilation if stage_idx == 5 else 1
        first_stride = 1 if idx == 0 or (stage_idx == 5 and dilation == 2) else 2
        stage_kargs = {
            "num_blocks": num_blocks_per_stage[idx],
            "stride_per_block": [first_stride] + [1] * (num_blocks_per_stage[idx] - 1),
            "in_channels": in_channels,
            "bottleneck_channels": bottleneck_channels,
            "out_channels": out_channels,
            "num_groups": num_groups,
            "norm": norm,
            "stride_in_1x1": stride_in_1x1,
            "dilation": dilation,
        }
        if stage_idx == trident_stage_idx:
            assert not deform_on_per_stage[
                idx
            ], "Not support deformable conv in Trident blocks yet."
            stage_kargs["block_class"] = TridentBottleneckBlock
            stage_kargs["num_branch"] = num_branch
            stage_kargs["dilations"] = branch_dilations
            stage_kargs["test_branch_idx"] = test_branch_idx
            stage_kargs.pop("dilation")
        elif deform_on_per_stage[idx]:
            stage_kargs["block_class"] = DeformBottleneckBlock
            stage_kargs["deform_modulated"] = deform_modulated
            stage_kargs["deform_num_groups"] = deform_num_groups
        else:
            stage_kargs["block_class"] = BottleneckBlock
        blocks = (
            make_trident_stage(**stage_kargs)
            if stage_idx == trident_stage_idx
            else ResNet.make_stage(**stage_kargs)
        )
        in_channels = out_channels
        out_channels *= 2
        bottleneck_channels *= 2

        if freeze_at >= stage_idx:
            for block in blocks:
                block.freeze()
        stages.append(blocks)
    return ResNet(stem, stages, out_features=out_features)

    Create a ResNet instance from config for TridentNet.

    Returns:
        ResNet: a :class:`ResNet` instance.
    
EVA-02/det/projects/TridentNet/tridentnet/trident_backbone.py