Option to access hydra configs from interpolation

[omry]

The hydra config node is removed from the tree before the program stats, which makes it impossible to rely on it at runtime.
The reason it's removed is that otherwise the user config would be very large even for a simple program.
That config however is still available at runtime through the HydraConfig singleston.

The idea here is to simply register a resolver function by the name of hydra, which would provide access to hydra config.

User usage would look like:

gpu_id: ${hydra:job.num}

This can be useful for accessing everything inside the hydra config, including parameters only available at runtime.

In addition, provide an option to call to_absolute_dir from interpolations.