test(video): add test cases for variable framerate videos

[XueSongTap]

Add comprehensive unit tests for variable framerate video decoding functionality:

• Test basic decoding of variable framerate content
• Verify exact seeking mode works correctly with variable frame rates
• Compare exact vs approximate seeking behavior
• Validate frame timing patterns match expected variable rate
• Test sequential decoding with monotonically increasing timestamps
• Ensure batch frame retrieval works with variable durations

These tests ensure the decoder properly handles videos where the frame rate
changes throughout the content, particularly during seek operations.

Related to #653

selt test:

(torchcodec-env) yxc@yxc-MS-7B89:~/code/2504/torchcodec_dev$ pytest test/test_variable_fps_video.py -v
=============================================== test session starts ===============================================
platform linux -- Python 3.10.17, pytest-8.3.5, pluggy-1.5.0 -- /home/yxc/.local/share/mamba/envs/torchcodec-env/bin/python3.10
cachedir: .pytest_cache
rootdir: /home/yxc/code/2504/torchcodec_dev
configfile: pyproject.toml
collected 12 items

test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_basic_decoding[cpu] PASSED               [  8%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_basic_decoding[cuda] SKIPPED (CUDA n...) [ 16%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_exact_seeking_mode[cpu] PASSED           [ 25%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_exact_seeking_mode[cuda] SKIPPED (CU...) [ 33%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_approximate_seeking_mode_behavior[cpu] PASSED [ 41%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_approximate_seeking_mode_behavior[cuda] SKIPPED [ 50%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_frame_timing_pattern[cpu] PASSED         [ 58%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_frame_timing_pattern[cuda] SKIPPED (...) [ 66%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_sequential_decoding[cpu] PASSED          [ 75%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_sequential_decoding[cuda] SKIPPED (C...) [ 83%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_frames_in_range[cpu] PASSED              [ 91%]
test/test_variable_fps_video.py::TestVariableFPSVideoDecoder::test_frames_in_range[cuda] SKIPPED (CUDA ...) [100%]

================================================ warnings summary =================================================
test/utils.py:99
  /home/yxc/code/2504/torchcodec_dev/test/utils.py:99: PytestCollectionWarning: cannot collect test class 'TestVideoStreamInfo' because it has a __init__ constructor (from: test/test_variable_fps_video.py)
    @dataclass

test/utils.py:93
  /home/yxc/code/2504/torchcodec_dev/test/utils.py:93: PytestCollectionWarning: cannot collect test class 'TestFrameInfo' because it has a __init__ constructor (from: test/test_variable_fps_video.py)
    @dataclass

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
==================================== 6 passed, 6 skipped, 2 warnings in 0.77s =====================================

due to lack of cuda device, cuda device test may need help

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