[Enhancement] Revise the interface of upfirdn2d function (#1195)

* revise the interface of upfirdn2d function

* adopt to_2tuple

mmcv/ops/upfirdn2d.py

@@ -99,6 +99,7 @@ import torch
 from torch.autograd import Function
 from torch.nn import functional as F
 
+from mmcv.utils import to_2tuple
 from ..utils import ext_loader
 
 upfirdn2d_ext = ext_loader.load_ext('_ext', ['upfirdn2d'])
@@ -249,20 +250,39 @@ def upfirdn2d(input, kernel, up=1, down=1, pad=(0, 0)):
     Args:
         input (Tensor): Tensor with shape of (n, c, h, w).
         kernel (Tensor): Filter kernel.
-        up (int, optional): Upsampling factor. Defaults to 1.
-        down (int, optional): Downsampling factor. Defaults to 1.
-        pad (tuple[int], optional): Padding for tensors, (x_pad, y_pad).
-            Defaults to (0, 0).
+        up (int | tuple[int], optional): Upsampling factor. If given a number,
+            we will use this factor for the both height and width side.
+            Defaults to 1.
+        down (int | tuple[int], optional): Downsampling factor. If given a
+            number, we will use this factor for the both height and width side.
+            Defaults to 1.
+        pad (tuple[int], optional): Padding for tensors, (x_pad, y_pad) or
+            (x_pad_0, x_pad_1, y_pad_0, y_pad_1). Defaults to (0, 0).
 
     Returns:
         Tensor: Tensor after UpFIRDn.
     """
     if input.device.type == 'cpu':
-        out = upfirdn2d_native(input, kernel, up, up, down, down, pad[0],
-                               pad[1], pad[0], pad[1])
+        if len(pad) == 2:
+            pad = (pad[0], pad[1], pad[0], pad[1])
+
+        up = to_2tuple(up)
+
+        down = to_2tuple(down)
+
+        out = upfirdn2d_native(input, kernel, up[0], up[1], down[0], down[1],
+                               pad[0], pad[1], pad[2], pad[3])
     else:
-        out = UpFirDn2d.apply(input, kernel, (up, up), (down, down),
-                              (pad[0], pad[1], pad[0], pad[1]))
+        _up = to_2tuple(up)
+
+        _down = to_2tuple(down)
+
+        if len(pad) == 4:
+            _pad = pad
+        elif len(pad) == 2:
+            _pad = (pad[0], pad[1], pad[0], pad[1])
+
+        out = UpFirDn2d.apply(input, kernel, _up, _down, _pad)
 
     return out