再次测试，将val数据集减少到一张图片，发现训练时显存占用3.8gb，val时占用15gb，并且首次val之后继续训练时显存仍然保持在13gb以上，第二次val时也炸了显存。

Added a PWM driver and a test file test_pwm.c. The test uses PWM to control the LED brightness, to check if the driver works correctly. Signed-off-by: XU HU 1337858472@qq.com ...