[ExecuTorch][Export][1/N] Export API pipeline re-architecture, making it composable

RFC: #12660

This diff introduces composable architecture for the export pipeline.

Changes:
1. Introduces notion of `PipelineArtifact` which encompasses stage artifacts and run context necessary to execute a stage and the pipeline.
1. Move all stages to new file to better architect the component
1. Export api will now accept `pipeline_stages` and honors the sequence if provided otherwise fallback to default sequence: `source_transform -> quantize -> torch.export -> to_edge_transform_and_lower -> to_executorch`
1. Validate if pipeline provided is valid, error out if not.
1. Add unittests to test pipeline sequence and stages

With this one can execute a partial pipeline, for example, one can just do `TORCH_EXPORT -> TO_EDGE_TRANSFORM_AND_LOWER -> TO_EXECUTORCH`

Current limitation:
1. `TORCH_EXPORT` stage is mandatory to avoid someone passing incorrect input such as `ExportedProgram` instead of nn.module, this enforcement will anchor the pipeline to have expected stages to run.
   - This limitation will be removed if we choose to add the support to take `ExportedProgram` as input.

Note:
1. this diff is large because I moved stages to separate file and added bunch of unittests but core functionality that is added can be reviewed in `export.py` file. CC: digantdesai
2. Export component was supposed to mimic stages design in tester and now it is.

Fixes: #12928

Differential Revision: [D79120574](https://our.internmc.facebook.com/intern/diff/D79120574/)

ghstack-source-id: 299106076
Pull Request resolved: #12936

Author: abhinaykukkadapu

export/TARGETS

@@ -15,7 +15,6 @@ runtime.python_library(
         "//caffe2:torch",
         "//executorch/exir/backend:backend_api",
         "//executorch/exir:pass_manager",
-        "//executorch/devtools/backend_debug:delegation_info",
         "//executorch/extension/export_util:export_util",
     ]
 )
@@ -31,11 +30,33 @@ runtime.python_library(
     ],
     deps = [
         ":recipe",
+        ":stages",
         "//executorch/runtime:runtime",
         ":recipe_registry"
     ]
 )
 
+
+runtime.python_library(
+    name = "stages",
+    srcs = [
+        "stages.py",
+    ],
+    visibility = [
+        "//executorch/...",
+        "@EXECUTORCH_CLIENTS",
+    ],
+    deps = [
+        ":recipe",
+        "//executorch/devtools/backend_debug:delegation_info",
+        "//executorch/exir/backend:backend_api",
+        "//executorch/exir:pass_manager",
+        "//caffe2:torch",
+        "//executorch/devtools/backend_debug:delegation_info",
+    ]
+)
+
+
 runtime.python_library(
     name = "lib",
     srcs = [
@@ -48,6 +69,7 @@ runtime.python_library(
     deps = [
         ":export",
         ":recipe",
+        ":stages",
         ":recipe_registry",
         ":recipe_provider"
     ],