Hi @Steve_Hu,
You can first try to zero pad the input:
# Given input = (B, 2, T, H, W)
input = torch.cat((input, torch.zeros(B, 1, T, H, W)), axis=1)

You can also modify the first layer and fine tune it with your 2 channels input. To modify it:
net = torchvision.models.video.r2plus1d_18()
r2.stem[0] = Conv3d(2, 45, kernel_size=(1, 7, 7), stride=(1, 2, 2), padding=(0, 3, 3), bias=False)