Make @graph_asset support explicit key (#16751)

## Summary & Motivation

`graph_asset`'s parameters are out-of-sync with `asset`'s on a structural level, and we do not have the discipline or the tests to keep them in sync. This PR adds an explicit `key` which is nice for users and would have made a recent PR of @johannkm's nicer, who had to do some contortions in #16612 around `key_prefix`. 

This diff also adds documentation for `key` on `@asset`.

An upstack PR will consolidate the logic in `@asset` and `@graph_asset` around this stuff into a single helper function, since it is pretty gross, tricky code.

## How I Tested These Changes

python_modules/dagster/dagster/_core/definitions/decorators/asset_decorator.py

@@ -199,6 +199,7 @@ def asset(
             execute in the decorated function after materializing the asset.
         non_argument_deps (Optional[Union[Set[AssetKey], Set[str]]]): Deprecated, use deps instead.
             Set of asset keys that are upstream dependencies, but do not pass an input to the asset.
+        key (Optional[CoeercibleToAssetKey]): The key for this asset. If provided, cannot specify key_prefix or name.
 
     Examples:
         .. code-block:: python
@@ -939,6 +940,7 @@ def graph_asset(
     backfill_policy: Optional[BackfillPolicy] = ...,
     resource_defs: Optional[Mapping[str, ResourceDefinition]] = ...,
     check_specs: Optional[Sequence[AssetCheckSpec]] = None,
+    key: Optional[CoercibleToAssetKey] = None,
 ) -> Callable[[Callable[..., Any]], AssetsDefinition]: ...
 
 
@@ -958,6 +960,7 @@ def graph_asset(
     backfill_policy: Optional[BackfillPolicy] = None,
     resource_defs: Optional[Mapping[str, ResourceDefinition]] = None,
     check_specs: Optional[Sequence[AssetCheckSpec]] = None,
+    key: Optional[CoercibleToAssetKey] = None,
 ) -> Union[AssetsDefinition, Callable[[Callable[..., Any]], AssetsDefinition]]:
     """Creates a software-defined asset that's computed using a graph of ops.
 
@@ -1001,6 +1004,7 @@ def graph_asset(
         auto_materialize_policy (Optional[AutoMaterializePolicy]): The AutoMaterializePolicy to use
             for this asset.
         backfill_policy (Optional[BackfillPolicy]): The BackfillPolicy to use for this asset.
+        key (Optional[CoeercibleToAssetKey]): The key for this asset. If provided, cannot specify key_prefix or name.
 
     Examples:
         .. code-block:: python
@@ -1033,13 +1037,24 @@ def slack_files_table():
             backfill_policy=backfill_policy,
             resource_defs=resource_defs,
             check_specs=check_specs,
+            key=key,
         )
     else:
+        if (name or key_prefix) and key:
+            raise DagsterInvalidDefinitionError(
+                "Cannot specify a name or key prefix for graph_asset when the key argument is"
+                " provided."
+            )
+
+        key = AssetKey.from_coercible(key) if key is not None else None
+
         key_prefix = [key_prefix] if isinstance(key_prefix, str) else key_prefix
         ins = ins or {}
         asset_name = name or compose_fn.__name__
         asset_ins = build_asset_ins(compose_fn, ins or {}, set())
-        out_asset_key = AssetKey(list(filter(None, [*(key_prefix or []), asset_name])))
+        out_asset_key = (
+            key if key else AssetKey(list(filter(None, [*(key_prefix or []), asset_name])))
+        )
 
         keys_by_input_name = {
             input_name: asset_key for asset_key, (input_name, _) in asset_ins.items()

python_modules/dagster/dagster_tests/asset_defs_tests/test_assets.py

@@ -41,6 +41,7 @@
 from dagster._core.definitions.asset_graph import AssetGraph
 from dagster._core.definitions.asset_spec import AssetSpec
 from dagster._core.definitions.auto_materialize_policy import AutoMaterializePolicy
+from dagster._core.definitions.decorators.asset_decorator import graph_asset
 from dagster._core.definitions.events import AssetMaterialization
 from dagster._core.definitions.result import MaterializeResult
 from dagster._core.errors import (
@@ -2070,3 +2071,73 @@ def asset_key_context_with_two_specs(context: AssetExecutionContext):
         match="Cannot call `context.asset_key` in a multi_asset with more than one asset",
     ):
         materialize([asset_key_context_with_two_specs])
+
+
+def test_graph_asset_explicit_coercible_key() -> None:
+    @op
+    def my_op() -> int:
+        return 1
+
+    # conversion
+    @graph_asset(key="my_graph_asset")
+    def _specified_elsewhere() -> int:
+        return my_op()
+
+    assert _specified_elsewhere.key == AssetKey(["my_graph_asset"])
+
+
+def test_graph_asset_explicit_fully_key() -> None:
+    @op
+    def my_op() -> int:
+        return 1
+
+    @graph_asset(key=AssetKey("my_graph_asset"))
+    def _specified_elsewhere() -> int:
+        return my_op()
+
+    assert _specified_elsewhere.key == AssetKey(["my_graph_asset"])
+
+
+def test_graph_asset_cannot_use_key_prefix_name_and_key() -> None:
+    @op
+    def my_op() -> int:
+        return 1
+
+    with pytest.raises(
+        DagsterInvalidDefinitionError,
+        match=(
+            "Cannot specify a name or key prefix for graph_asset when the key argument is provided"
+        ),
+    ):
+
+        @graph_asset(key_prefix="a_prefix", key=AssetKey("my_graph_asset"))
+        def _specified_elsewhere() -> int:
+            return my_op()
+
+        assert _specified_elsewhere  # appease linter
+
+    with pytest.raises(
+        DagsterInvalidDefinitionError,
+        match=(
+            "Cannot specify a name or key prefix for graph_asset when the key argument is provided"
+        ),
+    ):
+
+        @graph_asset(name="a_name", key=AssetKey("my_graph_asset"))
+        def _specified_elsewhere() -> int:
+            return my_op()
+
+        assert _specified_elsewhere  # appease linter
+
+    with pytest.raises(
+        DagsterInvalidDefinitionError,
+        match=(
+            "Cannot specify a name or key prefix for graph_asset when the key argument is provided"
+        ),
+    ):
+
+        @graph_asset(name="a_name", key_prefix="a_prefix", key=AssetKey("my_graph_asset"))
+        def _specified_elsewhere() -> int:
+            return my_op()
+
+        assert _specified_elsewhere  # appease linter