feat: add seq2seq forecasting training job (#1196)

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Fixes b/229909845 🦕

---

Adds a `SequenceToSequencePlusForecastingTrainingJob` to training jobs. This job has the exact same signature as `AutoMLForecastingTrainingJob`, but we are creating a separate job in case the two models diverge in the future.

The logic for `AutoMLForecastingTrainingJob` has been moved to a new abstract base class `_ForecastingTrainingJob`. The only things that differ between the seq2seq and automl training jobs that extend it are the `model_type` and `training_task_definition`.

google/cloud/aiplatform/__init__.py

@@ -63,6 +63,7 @@
     CustomPythonPackageTrainingJob,
     AutoMLTabularTrainingJob,
     AutoMLForecastingTrainingJob,
+    SequenceToSequencePlusForecastingTrainingJob,
     AutoMLImageTrainingJob,
     AutoMLTextTrainingJob,
     AutoMLVideoTrainingJob,
@@ -116,6 +117,7 @@
     "Model",
     "ModelEvaluation",
     "PipelineJob",
+    "SequenceToSequencePlusForecastingTrainingJob",
     "TabularDataset",
     "Tensorboard",
     "TensorboardExperiment",

google/cloud/aiplatform/schema.py

@@ -23,6 +23,7 @@ class definition:
         custom_task = "gs://google-cloud-aiplatform/schema/trainingjob/definition/custom_task_1.0.0.yaml"
         automl_tabular = "gs://google-cloud-aiplatform/schema/trainingjob/definition/automl_tabular_1.0.0.yaml"
         automl_forecasting = "gs://google-cloud-aiplatform/schema/trainingjob/definition/automl_time_series_forecasting_1.0.0.yaml"
+        seq2seq_plus_forecasting = "gs://google-cloud-aiplatform/schema/trainingjob/definition/seq2seq_plus_time_series_forecasting_1.0.0.yaml"
         automl_image_classification = "gs://google-cloud-aiplatform/schema/trainingjob/definition/automl_image_classification_1.0.0.yaml"
         automl_image_object_detection = "gs://google-cloud-aiplatform/schema/trainingjob/definition/automl_image_object_detection_1.0.0.yaml"
         automl_text_classification = "gs://google-cloud-aiplatform/schema/trainingjob/definition/automl_text_classification_1.0.0.yaml"