[Feature]: Add filter_link_idx to ContactForceSensor

[jsw7460]

What feature or enhancement are you proposing?

Add filter_link_idx to ContactForceSensor, mirroring the parameter ContactSensor in #2655.

ContactSensor can filter its boolean by counterpart link (blacklist), but ContactForceSensor reports the total contact force with no way to scope it.

Proposal: add filter_link_idx: array-like[int] to gs.sensors.ContactForce with the same semantics as gs.sensors.Contact: contacts whose counterpart link is in the list are excluded from the reported force.

Motivation

Wanted a ContactForceSensor that reports only the contact force from a specific subset of counterpart links (e.g. "force on the foot from the ground, excluding self-contact" or "force on the torso from anything except the robot itself"),

Potential Benefit

Reliable reward functions in RL

What is the expected outcome of the implementation work?

• gs.sensors.ContactForce accepts filter_link_idx: array-like[int], same semantics as gs.sensors.Contact.
• Reported force excludes contacts whose counterpart link is in the list, per-contact (correct even when the link touches multiple counterparts simultaneously).

Additional information

No response