next fieldview embedded: if domain is present, out fields don't have to have matching domains

[havogt]

Probably if an explicit domain is provided we only need to check that the provided domain is a subset of the out fields, i.e. something like

diff --git a/src/gt4py/next/embedded/operators.py b/src/gt4py/next/embedded/operators.py
index f50ace7..046eb57 100644
--- a/src/gt4py/next/embedded/operators.py
+++ b/src/gt4py/next/embedded/operators.py
@@ -90,10 +90,12 @@ def field_operator_call(op: EmbeddedOperator, args: Any, kwargs: Any):
         out = kwargs.pop("out")
         domain = kwargs.pop("domain", None)
 
-        flattened_out: tuple[common.Field, ...] = utils.flatten_nested_tuple((out,))
-        assert all(f.domain == flattened_out[0].domain for f in flattened_out)
-
-        out_domain = common.domain(domain) if domain is not None else flattened_out[0].domain
+        if domain:
+            out_domain = common.domain(domain)
+        else:
+            flattened_out: tuple[common.Field, ...] = utils.flatten_nested_tuple((out,))
+            assert all(f.domain == flattened_out[0].domain for f in flattened_out)
+            out_domain = flattened_out[0].domain
 
         new_context_kwargs["closure_column_range"] = _get_vertical_range(out_domain)