def _make_stem_layer(self, in_channels):
        """using a stride=1 conv as the stem in CSPDarknet."""
        # `stem_channels` equals to the `in_channels` in the first stage.
        stem_channels = self.arch['in_channels'][0]
        stem = ConvModule(
            in_channels=in_channels,
            out_channels=stem_channels,
            kernel_size=3,
            padding=1,
            norm_cfg=self.norm_cfg,
            act_cfg=self.act_cfg)
        return stem
using a stride=1 conv as the stem in CSPDarknet.
mmpretrain/models/backbones/cspnet.py