Add autocorr accessor to _BaseAccessor class

[suhaani-agarwal]

This is related to arviz-devs/arviz-plots#153.
Added an autocorr accessor to the _BaseAccessor class, similar to the existing hdi and ess accessors.

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📚 Documentation preview 📚: https://arviz-stats--65.org.readthedocs.build/en/65/

[aloctavodia]

Suggested change

	def autocorr(self, max_lag=None, dims=None, **kwargs):
	"""Compute autocorrelation for all variables in the dataset."""
	kwargs["max_lag"] = max_lag
	return self._apply("autocorr", dims=dims, **kwargs)
	def autocorr(self, dims=None, **kwargs):
	"""Compute autocorrelation for all variables in the dataset."""
	return self._apply("autocorr", dims=dims, **kwargs)

[aloctavodia]

compute everything and then handle max_lag when plotting.

[aloctavodia]

You also need to add something like this to datarray.py

def autocorr(self, da, dims=None):
    """Compute autocorrelation on DataArray input."""
    dims = validate_dims(dims)
    return apply_ufunc(
        self.array_class.autocorr,
        da,
        input_core_dims=[dims],
        output_core_dims=[dims],
    )

[suhaani-agarwal]

I have added the autocorr function in dataarray.py and made the changes you asked. @aloctavodia kindly review. Pls let me know if any further changes are required