fix p2p update error when disable_h2d_buffer is true

[ruizhang1230]

This MR fixes an issue introduced by #28.

The MR #28 modified the trigger condition of the P2P broadcast operation, requiring the H2D buffer. If the free GPU memory could not meet the condition for H2D operations, it would set the H2D buffer to None, preventing any communication buffer from being registered with the P2P store. This caused subsequent synchronization of the weight buffer to follow the wrong path, resulting in GPU memory synchronization errors.

In this fix MR, if H2D operations are available, the H2D buffer is registered with the P2P store when P2P operation are used to update weights; otherwise, the receive buffer is directly registered with the P2P store. And then, the corresponding copy_to_buffer operation is then executed through the right path.