ENH: n_outputs argument for HSGP.prior

[harrylojames]

Before

After

gp = pm.gp.HSGP(m=[25, 25], c=4.0, cov_func=cov_func)
f = gp.prior("f", X=X, n_outputs=10)

Context for the issue:

Hi,

First of all big fan of this package and the recent work on HSGP.

I am trying to use the HSGP class as a drop in replacement for pm.gp.Latent however the existing code uses n_outputs.

Would it be possible to implement this argument for HSGP.prior?

[welcome]

]
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