[Add README] add backend/README.md for vendor to fit (FlagOpen#387)

* fix bug

* fix test bug

* add backend/README.md

* fix

* modify

* modify

* modify

src/flag_gems/runtime/backend/README.md

@@ -0,0 +1,67 @@
+## Multiple backend adaptations
+### Introduction
+The `flag_gems` operator library provides the ability to access multiple backends.  If you are a chip vendor and wish to integrate  `your flag_gems code` into our official main branch, you simply need to follow these steps to complete the process.
+
+#### step 1:
+Create a folder named after your vendor in the  `FlagGems/src/flag_gems/runtime/backend directory`, following the pattern `_vendorname`. For example, you can refer to the structure of  `FlagGems/src/flag_gems/runtime/backend/_nvidia`.
+
+#### step 2:
+Create the necessary files, including but not limited to `__init__.py`, `heuristics_config_utils.py`, `tune_configs.yaml`, as well as a folder named  `ops`. This is an example under _nvidia file:
+```
+├── __init__.py
+├── heuristics_config_utils.py
+├── ops
+│   ├── __init__.py
+│   ├── add.py
+│   └── gelu.py
+└── tune_configs.yaml
+```
+
+##### step 2.1  `__init__.py`
+
+You can copy `FlagGems/src/flag_gems/runtime/backend/_nvidia/__init__.py` and the  ***only change***  you need to make is to configure the `VendorInfoBase` class
+```
+VendorInfoBase(
+    vendor_name="xxx", device_name="xxx", device_query_cmd="xxx"
+)
+```
+- `vendor_name` is your vendorname, like `nvidia`
+
+- `device_name` is your devicename, like `cuda`
+
+- `device_query_cmd` is a command that can only be successfully executed on your vendor's device, like `nvidia-smi`
+
+##### step 2.2  `heuristics_config_utils.py`
+
+You should configure  `triton.heuristics` params in `FlagGems/src/flag_gems/runtime/backend/_vendorname/heuristics_config_utils.py`and you can  refer to `FlagGems/src/flag_gems/runtime/backend/_nvidia/heuristics_config_utils.py`
+
+##### step 2.3  `tune_configs.yaml`
+You should configure  `triton.autotune` params in `FlagGems/src/flag_gems/runtime/backend/_vendorname/tune_configs.yaml` and you can refer to `FlagGems/src/flag_gems/runtime/backend/_nvidia/tune_configs.yaml`
+
+##### step 2.4  `ops`
+The `ops` directory is where `vendor-customized operator` implementations are stored. For instance, if you want to create a custom `add operation`, you should place the implementation in `ops/add.py`. Following that, you should configure ops/__init__.py accordingly.
+```python
+from backend_utils import Autograd
+
+from . import add, gelu
+
+def get_specific_ops():
+    return (
+        ("add.Tensor", add.add, Autograd.disable),
+        ("gelu", gelu.gelu, Autograd.enable),
+    )
+
+
+def get_unused_ops():
+    return ("cumsum", "cos")
+```
+- The `get_specific_ops` function is designed to retrieve vendor-customized operators. If this feature is not required, the function can return an `empty tuple: ()`. An item such as `"add.Tensor"`, `add.add`, `Autograd.disable` is used to describe operator registration details.
+
+
+    - `"add.Tensor"` denotes the name of the Aten operation you wish to replace.
+
+    - `add.add` represents the implementation of the add operator using Triton.
+
+    - `Autograd.enable/Autograd.disable` indicates whether the operator supports backward computation (gradient calculation) or not.
+
+- The `get_unused_ops()` function is intended to obtain a list of operations that vendors prefer users not to utilize. If this functionality is not needed, the function can also return an `empty tuple: ()`. An item such as `"cumsum"`  is a string type represents the name of op you don't want users to use it.