Create FIX_LOWERING_FOR_CORE_ATEN_OPS.md (#5909)

FIX_LOWERING_FOR_CORE_ATEN_OPS.md

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+In order for PyTorch/XLA to support the PyTorch core ATen opset, it requires lowering each core ATen op in PyTorch/XLA. Note that this document will serve as a guide for fixing these lowering for core aten opset, specifically looking at [test_core_aten_ops.py test](https://github.com/pytorch/xla/blob/master/test/test_core_aten_ops.py). This guide will **not** cover how to lower an op in PyTorch/XLA, please refer our [op lowering guide](https://github.com/pytorch/xla/blob/master/OP_LOWERING_GUIDE.md) for this. 
+
+Let's go back and take a closer look at the [test_core_aten_ops.py test](https://github.com/pytorch/xla/blob/master/test/test_core_aten_ops.py), which is the source of truth to verify and correctness of these lowerings. The core of this test file is the `run_export_and_compare` at https://github.com/pytorch/xla/blob/master/test/test_core_aten_ops.py#L28. Each op unit test initializes the input and passes the op as a function and its inputs. The `run_export_and_compare` has multiple subtests that have the following structure: 
+- `torch_eval`
+   - `torch_xla_eval`
+     - `torch_xla_diff`
+   - `can_export`
+     - `can_convert_to_stablehlo`
+       - `stablehlo_can_run`
+         - `stablehlo_diff`
+
+Below we'll describe what each of these subtests mean and give some recommendations on fixing it.
+
+### `torch_eval`
+
+This subtest directly calls torch version of the op with the given inputs. If the unit test fails in this subtest, this implies that torch there is a problem with the unit test itself. One common reason might be due to inputs (or types of inputs) not being compatible with the op. We recommend you to look at the official torch documentation of the corresponding op to ensure that that unit tests are passing valid inputs to the op.
+
+### `torch_eval_xla`
+
+This subtest calls the torch_xla version of the op. If you've made changes to lower the op and this subtest fails, this means there may be something wrong with the lowering. We recommend you to take another look at our [op lowering guide](https://github.com/pytorch/xla/blob/master/OP_LOWERING_GUIDE.md). If you're unable to debug further, feel free to leave a comment in your assigned GitHub issue.
+
+### `torch_xla_diff`
+
+This subtest compares the output of the op between torch and torch_xla. If this subtest fails, it implies that your lowering runs successfully but contains a bug and/or logical error. We recommend you to review your lowering code. And again, feel free to leave a comment in your assigned GitHub issue if you're blocked and/or unable to debug further.
+
+### `can_export`, `can_convert_to_stablehlo`, `stablehlo_can_run`, `stablehlo_diff`
+
+These subtests are related to `export` and `stablehlo`. If the lowering is complete and the above `torch_*` subtests all succeed, it is highly likely that these tests will also succeed.