This PR improves the CMake build system for the C++ API of flashinfer,
making it more modular, easier to configure, and overall easier to
integrate into downstream projects. The changes introduce a proper
component-based installation for different usage scenarios for the C++
API, adds CMake-based dependency management, and generally offers a
cleaner build organization.

The C++ sources, tests, and benchmarks are now reorganized inside a
`libflashinfer` directory. The original directories are left unmodified.

flashinfer/
├── CMakeLists.txt
├── include/
│   └── flashinfer/
└── src/

flashinfer/
├── CMakeLists.txt
├── cmake/
│   └── utils/
│   │   └── ConfigureTargets.cmake
│   ├── Config.cmake.in
│   ├── Dependencies.cmake
│   ├── Options.cmake
├── libflashinfer/
│   ├── CMakeLists.txt
│   ├── include/
│   │   └── flashinfer/
│   │       ├── attention/
│   │       └── distributed/
│   │       └── gemm/
│   ├── tests/
│   │   ├── CMakeLists.txt          # Consistent test configuration
│   │   └── fp8/                    # FP8-specific tests
│   ├── utils/
│   │   └── fp8/                    # FP8 utility files
│   └── benchmarks/
│       ├── CMakeLists.txt          # Standardized benchmark setup
│       └── fp8/                    # FP8-specific benchmarks

- Separated flashinfer CMake option definitions into distinct files for
better organization
- Created utility modules for configuring tests and benchmarks
consistently
- Consolidated dependency management in a single location
- Added a flashinferconfig for downstream projects to be able to do
`find_package(flashinfer)` in their CMake scripts.

- The C++ API is broken down into multiple components: **Headers,
Kernels, TVMBinding, Distributed.**
- Components can be configured and built separately
- Ex1. Configuring the C++ API to build AOT kernels, C++ unit tests and
benchmarks
    ```bash
cmake .. -GNinja -DFLASHINFER_CUDA_ARCHITECTURES=80
-DFLASHINFER_BUILD_KERNELS=ON -DFLASHINFER_UNITTESTS=ON
-DFLASHINFER_CXX_BENCHMARKS=ON
-DFLASHINFER_CUTLASS_DIR=../3rdparty/cutlass
-DCMAKE_INSTALL_PREFIX=~/devel/install
    ```
    - Ex2. Build various components as needed
    ```bash
        # Build the test_single_decode unit tests
        cmake --build . --target test_single_decode
        # Build all unit tests (broken right now)
        cmake --build . --target build_tests
        # Build bench_single_decode
        cmake --build . --target bench_single_decode
        # Build all benchmarks (broken right now)
        cmake --build . --target build_benchmarks
    ```
   - Ex3. Install components
       ```bash
           cmake --build . --target install
       ```
- Added CMake targets to generate sources based on the same logic as in
the current `setup.py`. Generated sources are now placed under the
`CMAKE_CURRENT_BINARY_DIR`

   - Ex4. Install components
       ```bash
           cmake --build . --target generate_kernels
       ```

These changes are primarily meant to streamline the changes to add new
CMake config options for a proposed HIP back end. However, it is
foreseeable to build upon these changes to convert the build system to a
fully unified CMake-based using `scikit-build` and use a single
configuration system for both C++ and Python APIs.

In this PR I remove the `libtorch` dependency and removed
`test_page.cpp`. `test_page.cpp` is the only unit test that uses
libtorch. However, we also have a pytest for testing page. We will use
that for validation.

Removing the libtorch dependency will help us speed docker builds and
remove additional dependencies.


```Test project /root/flashinfer/libflashinfer/tests/hip/build
    Start 1: MathTest
1/8 Test #1: MathTest ............................   Passed    0.31 sec
    Start 2: PosEncTest
2/8 Test #2: PosEncTest ..........................   Passed    0.31 sec
    Start 3: CascadeTest
3/8 Test #3: CascadeTest .........................   Passed  1369.12 sec
    Start 4: SingleDecodeTest
4/8 Test #4: SingleDecodeTest ....................   Passed  7726.35 sec
    Start 5: BatchDecodeTest
5/8 Test #5: BatchDecodeTest .....................   Passed  811.61 sec
    Start 6: test_mfma_fp32_16x16x16fp16
6/8 Test #6: test_mfma_fp32_16x16x16fp16 .........   Passed    0.30 sec
    Start 7: test_transpose_4x4_half_registers
7/8 Test #7: test_transpose_4x4_half_registers ...   Passed    0.28 sec
    Start 8: test_rowsum
8/8 Test #8: test_rowsum .........................   Passed    0.27 sec

100% tests passed, 0 tests failed out of 8
```

* Updates to permuted_smem.cuh

    - Updated the upcast_size function to always require a VectorWidthBits tparam
    - Added load_fragment, load_fragment_and_quad_transpose, store_fragment, load_vector_async,
      store_64b, and store_vector fuctions.