[ENH] TimeXer model from thuml (#1797)

### Description

This PR works on #1793 and aims to align and implement the `TimeXer` model within PTF's design.

Author: PranavBhatP

pytorch_forecasting/models/__init__.py

@@ -19,6 +19,7 @@
     TemporalFusionTransformer,
 )
 from pytorch_forecasting.models.tide import TiDEModel
+from pytorch_forecasting.models.timexer import TimeXer
 
 __all__ = [
     "NBeats",
@@ -37,4 +38,5 @@
     "MultiEmbedding",
     "DecoderMLP",
     "TiDEModel",
+    "TimeXer",
 ]

pytorch_forecasting/models/timexer/__init__.py

@@ -0,0 +1,31 @@
+"""
+TimeXer model for forecasting time series.
+"""
+
+from pytorch_forecasting.models.timexer._timexer import TimeXer
+from pytorch_forecasting.models.timexer._timexer_metadata import TimeXerMetadata
+from pytorch_forecasting.models.timexer.sub_modules import (
+    AttentionLayer,
+    DataEmbedding_inverted,
+    Encoder,
+    EncoderLayer,
+    EnEmbedding,
+    FlattenHead,
+    FullAttention,
+    PositionalEmbedding,
+    TriangularCausalMask,
+)
+
+__all__ = [
+    "TimeXer",
+    "TriangularCausalMask",
+    "FullAttention",
+    "AttentionLayer",
+    "DataEmbedding_inverted",
+    "PositionalEmbedding",
+    "FlattenHead",
+    "EnEmbedding",
+    "Encoder",
+    "EncoderLayer",
+    "TimeXerMetadata",
+]