chore: import upstream snapshot with attribution

tensorflow/lite/kernels/variants/README.md

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+# TfLite Variants
+
+Variant tensors in TensorFlow wrap and store arbitrary C++ objects within their
+data members. Common usage regards non-trivial and potentially referential
+buffer semantics (TensorLists and DataSets being cannonical examples).
+
+This directory contains implementations for these containers
+and kernels for the tflite runtime.
+Currently tflite supplies performant custom kernels for a subset of
+`tf.list_ops`.
+Also refer to [variant tensor in the tflite common api](https://github.com/tensorflow/tensorflow/blob/61c76427561a46d03605370fc685d810c1c3e717/tensorflow/lite/core/c/common.h#L1320C9-L1320C9)
+, [tensor list legalization](https://github.com/tensorflow/tensorflow/blob/d36fb81bc1ef258d5024b791d61cdd5136ca09af/tensorflow/compiler/mlir/lite/transforms/legalize_tensorlist.cc)
+, and [end to end python tests](https://github.com/tensorflow/tensorflow/blob/d36fb81bc1ef258d5024b791d61cdd5136ca09af/tensorflow/lite/kernels/variants/py/end_to_end_test.py)
+for example usage.
+
+**api**
+
+* `./py/register_list_ops_py.py` : Bindings for registering ops in python.
+* `./list_ops_lib` : Include for tensorlist kernel registrations.
+* `./register_list_ops` : Register all kernels with op resolver in C++.
+
+**implementations**
+
+* `./list_kernels/` : Custom `TensorList*` kernels for tflite.
+* `./tensor_array` : A variable length array of reference counted `TfLiteTensor`
+.
+
+**tests**
+
+* `/list_ops_subgraph_test` : Multi-Op tests through C++ api.
+* `/py/end_to_end_test.py` : Tests through python api and compare to `tf.list_ops`.