[quant][refactor] Remove the base class from __all__

In general, if we are expecting the users to use the base class,
such as `_ConvNd`, we should rename it to something like
`BaseConv`. However, because this base class is only used inside of the
AO packages, there is no need to expose it to the users.

Test Plan:

```
python test/test_quantization.py
python test/test_module_init.py
```

Pull Request resolved: https://github.com/pytorch/pytorch/pull/77344

Approved by: https://github.com/jerryzh168

test/test_module_init.py

@@ -365,7 +365,6 @@ def generate_tests(test_cls, constructor_arg_db):
         torch.nn.Module,
         torch.nn.Container,  # deprecated
         torch.nn.NLLLoss2d,  # deprecated
-        torch.nn.quantized._ConvNd,  # base class in __all__ for some reason
         # TODO: Remove these 2 from this list once the ASan issue is fixed.
         # See https://github.com/pytorch/pytorch/issues/55396
         torch.nn.quantized.Embedding,

torch/nn/quantized/modules/__init__.py

@@ -6,7 +6,7 @@ from .dropout import Dropout
 from .batchnorm import BatchNorm2d, BatchNorm3d
 from .normalization import LayerNorm, GroupNorm, InstanceNorm1d, \
     InstanceNorm2d, InstanceNorm3d
-from .conv import _ConvNd, Conv1d, Conv2d, Conv3d
+from .conv import Conv1d, Conv2d, Conv3d
 from .conv import ConvTranspose1d, ConvTranspose2d, ConvTranspose3d
 from .linear import Linear
 from .embedding_ops import Embedding, EmbeddingBag
@@ -91,7 +91,6 @@ class DeQuantize(torch.nn.Module):
 __all__ = [
     'BatchNorm2d',
     'BatchNorm3d',
-    '_ConvNd',
     'Conv1d',
     'Conv2d',
     'Conv3d',

torch/testing/_internal/common_modules.py

@@ -31,7 +31,6 @@ MODULES_TO_SKIP: Set[Type] = {
     torch.nn.Module,  # abstract base class
     torch.nn.Container,  # deprecated
     torch.nn.NLLLoss2d,  # deprecated
-    torch.nn.quantized.modules._ConvNd,  # abstract base class
     torch.nn.quantized.MaxPool2d,  # aliases to nn.MaxPool2d
 }