Issue in calculation of log_probs in Sampler

[hyeok9855]

This issue arises when I implement the LocalSearchSampler, especially the Metropolis-Hastings (MH) rule.

In short, we need both the log_pbs and log_pfs of each trajectory $\tau_{bwd}=(x\to\cdots s' \cdots \to s_0)$ and $\tau_{recon}=(s_0 \to \cdots s' \cdots \to x')$ for MH (see this!).

There are two issues here.

1. Trajectory reversion may be needed for simpler computation of log_pfs($\tau_{bwd}$) - Check the related comment
2. It's hard to calculate log_pbs and log_pfs within current structure (TrajectoryBasedGFlowNet has method get_pfs_and_pbs, but Sampler can't access to it) - Check the related comments.