feat(vnet): convert dropout_prob to a tuple (#6768)

Fixes #6116 

### Description

Converts dropout probability from a Optional float to a Optional tuple
of floats.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [x] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

---------

Signed-off-by: Saurav Maheshkar <sauravvmaheshkar@gmail.com>

monai/networks/nets/vnet.py

@@ -16,6 +16,7 @@
 
 from monai.networks.blocks.convolutions import Convolution
 from monai.networks.layers.factories import Act, Conv, Dropout, Norm, split_args
+from monai.utils import deprecated_arg
 
 __all__ = ["VNet"]
 
@@ -133,7 +134,7 @@ def __init__(
         out_channels: int,
         nconvs: int,
         act: tuple[str, dict] | str,
-        dropout_prob: float | None = None,
+        dropout_prob: tuple[float | None, float] = (None, 0.5),
         dropout_dim: int = 3,
     ):
         super().__init__()
@@ -144,8 +145,8 @@ def __init__(
 
         self.up_conv = conv_trans_type(in_channels, out_channels // 2, kernel_size=2, stride=2)
         self.bn1 = norm_type(out_channels // 2)
-        self.dropout = dropout_type(dropout_prob) if dropout_prob is not None else None
-        self.dropout2 = dropout_type(0.5)
+        self.dropout = dropout_type(dropout_prob[0]) if dropout_prob[0] is not None else None
+        self.dropout2 = dropout_type(dropout_prob[1])
         self.act_function1 = get_acti_layer(act, out_channels // 2)
         self.act_function2 = get_acti_layer(act, out_channels)
         self.ops = _make_nconv(spatial_dims, out_channels, nconvs, act)
@@ -206,8 +207,9 @@ class VNet(nn.Module):
             The value should meet the condition that ``16 % in_channels == 0``.
         out_channels: number of output channels for the network. Defaults to 1.
         act: activation type in the network. Defaults to ``("elu", {"inplace": True})``.
-        dropout_prob: dropout ratio. Defaults to 0.5.
-        dropout_dim: determine the dimensions of dropout. Defaults to 3.
+        dropout_prob_down: dropout ratio for DownTransition blocks. Defaults to 0.5.
+        dropout_prob_up: dropout ratio for UpTransition blocks. Defaults to (0.5, 0.5).
+        dropout_dim: determine the dimensions of dropout. Defaults to (0.5, 0.5).
 
             - ``dropout_dim = 1``, randomly zeroes some of the elements for each channel.
             - ``dropout_dim = 2``, Randomly zeroes out entire channels (a channel is a 2D feature map).
@@ -216,15 +218,29 @@ class VNet(nn.Module):
             According to `Performance Tuning Guide <https://pytorch.org/tutorials/recipes/recipes/tuning_guide.html>`_,
             if a conv layer is directly followed by a batch norm layer, bias should be False.
 
+    .. deprecated:: 1.2
+        ``dropout_prob`` is deprecated in favor of ``dropout_prob_down`` and ``dropout_prob_up``.
+
     """
 
+    @deprecated_arg(
+        name="dropout_prob",
+        since="1.2",
+        new_name="dropout_prob_down",
+        msg_suffix="please use `dropout_prob_down` instead.",
+    )
+    @deprecated_arg(
+        name="dropout_prob", since="1.2", new_name="dropout_prob_up", msg_suffix="please use `dropout_prob_up` instead."
+    )
     def __init__(
         self,
         spatial_dims: int = 3,
         in_channels: int = 1,
         out_channels: int = 1,
         act: tuple[str, dict] | str = ("elu", {"inplace": True}),
-        dropout_prob: float = 0.5,
+        dropout_prob: float | None = 0.5,  # deprecated
+        dropout_prob_down: float | None = 0.5,
+        dropout_prob_up: tuple[float | None, float] = (0.5, 0.5),
         dropout_dim: int = 3,
         bias: bool = False,
     ):
@@ -236,10 +252,10 @@ def __init__(
         self.in_tr = InputTransition(spatial_dims, in_channels, 16, act, bias=bias)
         self.down_tr32 = DownTransition(spatial_dims, 16, 1, act, bias=bias)
         self.down_tr64 = DownTransition(spatial_dims, 32, 2, act, bias=bias)
-        self.down_tr128 = DownTransition(spatial_dims, 64, 3, act, dropout_prob=dropout_prob, bias=bias)
-        self.down_tr256 = DownTransition(spatial_dims, 128, 2, act, dropout_prob=dropout_prob, bias=bias)
-        self.up_tr256 = UpTransition(spatial_dims, 256, 256, 2, act, dropout_prob=dropout_prob)
-        self.up_tr128 = UpTransition(spatial_dims, 256, 128, 2, act, dropout_prob=dropout_prob)
+        self.down_tr128 = DownTransition(spatial_dims, 64, 3, act, dropout_prob=dropout_prob_down, bias=bias)
+        self.down_tr256 = DownTransition(spatial_dims, 128, 2, act, dropout_prob=dropout_prob_down, bias=bias)
+        self.up_tr256 = UpTransition(spatial_dims, 256, 256, 2, act, dropout_prob=dropout_prob_up)
+        self.up_tr128 = UpTransition(spatial_dims, 256, 128, 2, act, dropout_prob=dropout_prob_up)
         self.up_tr64 = UpTransition(spatial_dims, 128, 64, 1, act)
         self.up_tr32 = UpTransition(spatial_dims, 64, 32, 1, act)
         self.out_tr = OutputTransition(spatial_dims, 32, out_channels, act, bias=bias)