Add default args for _aten_conv2d (#9623)

Add default args for _aten_conv2d, which would otherwise fail in the
following code snippet

```python
import torch
from torch.export import export_for_training
import torchax
from torchax import interop
from torch.utils import _pytree as pytree
import jax
from torchax.ops import mappings

class Simple(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.conv1 = torch.nn.Conv2d(3, 64, kernel_size=4, bias=False)

    def forward(self, x):
        x = self.conv1(x)
        return x
    

model = Simple()

exported = export_for_training(model, (torch.randn(1, 3, 224, 224),))

def make_shape_struct(x):
    return jax.ShapeDtypeStruct(x.shape, mappings.t2j_dtype(x.dtype))


def map_nth(v, i):
    def f(t):
        if isinstance(t, torch.Tensor):
            return t[i : i + 1]
        return t

    return pytree.tree_map(f, v)


env = torchax.default_env()
with env:
    model = exported.module().to("jax")

    def func_to_export(x):
        # hard code weights in model
        return model(x)

    example_inputs_jax = pytree.tree_map_only(
        torch.Tensor, lambda x: x.to("jax"), map_nth(exported.example_inputs, 0)
    )

    res = jax.jit(interop.jax_view(func_to_export)).lower(*example_inputs_jax[0])

# TypeError: _aten_conv2d() missing 5 required positional arguments: 'bias', 'stride', 'padding', 'dilation', and 'groups'
```
cc @qihqi

Author: hsjts0u

torchax/torchax/ops/jaten.py

@@ -1017,11 +1017,11 @@ def _aten_bucketize(input,
 def _aten_conv2d(
     input,
     weight,
-    bias,
-    stride,
-    padding,
-    dilation,
-    groups,
+    bias=None,
+    stride=[1, 1],
+    padding=[0, 0],
+    dilation=[1, 1],
+    groups=1,
 ):
   return _aten_convolution(
       input,