[IR] Create readme (#1437)

Initial readme for the IR. Describes the file structure.

onnxscript/ir/README.md

@@ -0,0 +1,21 @@
+# ONNX IR
+
+An in-memory IR that supports the full ONNX spec, designed for graph construction, analysis and transformation.
+
+## Features ✨
+
+- Full ONNX spec support: all valid models representable by ONNX protobuf, and a subset of invalid models (so you can load and fix them).
+- Low memory footprint: mmap'ed external tensors; unified interface for ONNX TensorProto, Numpy arrays and PyTorch Tensors etc. No tensor size limitation. Zero copies.
+- Straightforward access patterns: Access value information and traverse the graph topology at ease.
+- Robust mutation: Create as many iterators as you like on the graph while mutating it.
+- Speed: Performant graph manipulation, serialization/deserialization to Protobuf.
+- Pythonic and familiar APIs: Classes define Pythonic apis and still map to ONNX protobuf concepts in an intuitive way.
+
+## Code Organization 🗺️
+
+- [`_protocols.py`](_protocols.py): Interfaces defined for all entities in the IR.
+- [`_core.py`](_core.py): Implementation of the core entities in the IR, including `Model`, `Graph`, `Node`, `Value`, and others.
+- [`_enums.py`](_enums.py): Definition of the type enums that correspond to the `DataType` and `AttributeType` in `onnx.proto`.
+- [`_name_authority.py`](_name_authority.py): The authority for giving names to entities in the graph, used internally.
+- [`_linked_list.py`](_linked_list.py): The data structure as the node container in the graph that supports robust iteration and mutation. Internal.
+- [`_metadata.py`](_metadata.py): Metadata store for all entities in the IR.

onnxscript/ir/_linked_list.py

@@ -96,6 +96,7 @@ class DoublyLinkedSet(Generic[T], Sequence[T]):
 
     def __init__(self, values: Iterable[T] | None = None) -> None:
         # Using the root node simplifies the mutation implementation a lot
+        # The list is circular. The root node is the only node that is not a part of the list values
         root_ = _LinkBox(self, None)
         self._root: _LinkBox = root_
         self._length = 0