[1/n] Reorganize sparsity module to separate weight and attention sparsity (#517)

## What does this PR do?

**Type of change:** ? <!-- Use one of the following: Bug fix, new
feature, new example, new tests, documentation. -->

**Overview:** ?
Reorganizes the `modelopt/torch/sparsity` module to prepare for adding
attention sparsity support. All existing weight sparsity functionality
(source code and tests) is moved into a dedicated `weight_sparsity`
subdirectory while maintaining full backward compatibility.

```
# These imports still work exactly as before
from modelopt.torch.sparsity import sparsify, magnitude_prune
from modelopt.torch.sparsity.mode import SparsityMode
```

## Usage
<!-- You can potentially add a usage example below. -->

```python
# Add a code snippet demonstrating how to use this
```

## Testing
<!-- Mention how have you tested your change if applicable. -->

## Before your PR is "*Ready for review*"
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`Draft` PR. -->

- **Make sure you read and follow [Contributor
guidelines](https://github.com/NVIDIA/TensorRT-Model-Optimizer/blob/main/CONTRIBUTING.md)**
and your commits are signed.
- **Is this change backward compatible?**: Yes/No <!--- If No, explain
why. -->
- **Did you write any new necessary tests?**: Yes/No
- **Did you add or update any necessary documentation?**: Yes/No
- **Did you update
[Changelog](https://github.com/NVIDIA/TensorRT-Model-Optimizer/blob/main/CHANGELOG.rst)?**:
Yes/No <!--- Only for new features, API changes, critical bug fixes or
bw breaking changes. -->

## Additional Information
<!-- E.g. related issue. -->

Signed-off-by: Kai Xu <kaix@nvidia.com>

Author: kaix-nv

modelopt/torch/sparsity/__init__.py

@@ -13,7 +13,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-"""API for sparsification algorithms."""
+"""API for sparsification algorithms.
 
-from . import mode, module, plugins
-from .sparsification import *
+This module provides access to both weight sparsity and attention sparsity algorithms.
+For backward compatibility, weight sparsity APIs are re-exported at the module level.
+"""
+
+# Import weight sparsity for backward compatibility
+from .weight_sparsity import mode, module, plugins
+from .weight_sparsity.sparsification import *

modelopt/torch/sparsity/weight_sparsity/__init__.py

@@ -0,0 +1,23 @@
+# SPDX-FileCopyrightText: Copyright (c) 2024 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""API for weight sparsification algorithms."""
+
+from . import mode, module, plugins
+
+# Explicitly expose commonly used items
+from .mode import SparsityModeRegistry
+from .module import SparseModule, SpDMRegistry
+from .sparsification import *