[IO] Added support for import and export of MJCF files

[ShravanTata]

We are right now limited to only load models using SDF file format. It would be beneficial to extend this to MJCF files. Since, MJCF files have additional features that are used in practice than an SDF it is important to consider how we handle those options in FARMS. For example, the presence the mujoco simulation options which are often described in sim options

[jarreguit]

I agree, being able to switch between formats will be useful for importing available models designed by others. In practice, to facilitate development and maintenance, we still need to choose a main format for storing our models, which we can then write importers and exporters to other formats.

The current format is indeed SDF due to historical reasons (FARMS originally focused around Gazebo and Pybullet), although switching to USD might be of interest as Nvidia and Deepmind have been considering it. What do you think?

For now, this issue should probably be limited to directly extending our current SDF-based master format to handle extension to MJCF with an importer and an exporter, based on the work of the mjcf.py in farms_mujoco. Replacing SDF by USD as a master format could be opened as a separate issue.

Note that regardless of the master format we select, that format should permit a user to create a simulation within any physics engine FARMS supports based on all its contents such that we can properly transfer experiments between engines.

[ShravanTata]

I agree, being able to switch between formats will be useful for importing available models designed by others. In practice, to facilitate development and maintenance, we still need to choose a main format for storing our models, which we can then write importers and exporters to other formats.

The current format is indeed SDF due to historical reasons (FARMS originally focused around Gazebo and Pybullet), although switching to USD might be of interest as Nvidia and Deepmind have been considering it. What do you think?

Yes, USD is a promising format and potentially being the industry standard. However, at this point I have very minimal experience with it. I am planning to learn it for another project and will be able to better judge it's fit for FARMS as the main internal data format after that. As of now, from what I have read it should handle the different physics engines we support.

For now, this issue should probably be limited to directly extending our current SDF-based master format to handle extension to MJCF with an importer and an exporter, based on the work of the mjcf.py in farms_mujoco. Replacing SDF by USD as a master format could be opened as a separate issue.

Agreed! The current setup should easily facilitate loading and simulating the MJCF file format. We should start using MjSpec instead of dm_control while we are at it (see https://mujoco.readthedocs.io/en/stable/programming/modeledit.html#overview). I worked with MjSpec and it is really handy and easy. However, without a main FARMS internal format we will break the blender pipeline.

Note that regardless of the master format we select, that format should permit a user to create a simulation within any physics engine FARMS supports based on all its contents such that we can properly transfer experiments between engines.

Yes, agreed. This is very important for FARMS.