Add "forward chamfer distance" in pytorch3d.loss.chamfer

## 🚀 Feature

Forward chamfer distance (chamfer distance only from source to target)

## Motivation

Currently, in pytorch3d.loss.chamfer, the chamfer_distance function can returns "cham_dist = cham_x + cham_y". However, in many cases, we only need "forward chamfer distance", i.e., "cham_x" in the code.

## Pitch

It would be great to have a parameter (i.e., direction = "forward" or "bidirectional") to choose which to return (cham_x or cham_x + cham_y).