[builder] correct readme (#2375)

* [example] add google doc for benchmark results of GPT

* add tencet doc

* [example] gpt, shard init on all processes

* polish comments

* polish code

* [builder] update readme

README.md

@@ -277,10 +277,11 @@ pip install -r requirements/requirements.txt
 pip install .
 ```
 
-If you don't want to install and enable CUDA kernel fusion (compulsory installation when using fused optimizer):
+By default, we do not compile CUDA/C++ kernels. ColossalAI will build them during runtime.
+If you want to install and enable CUDA kernel fusion (compulsory installation when using fused optimizer):
 
 ```shell
-NO_CUDA_EXT=1 pip install .
+CUDA_EXT=1 pip install .
 ```
 
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