Add PyTorch-native implementation of custom layers

[WoosukKwon]

This PR adds the pytorch-native implementation of the custom ops to the _forward() method of the corresponding layers. While the _forward() method will be never called at the moment, this change will bring the following benefits:

1. It helps users understand the custom layers
2. It can be used for debugging. Users can simply replace forward with _forward and inspect the intermediate values.
3. It reduces some redundant code in the tests.
4. In the future, we can use pytorch compiler to generate fused kernel from the pytorch code, instead of using the custom ops.