Move developer guide file to a folder

Summary:
Moved the dev api guide to a folder and we plan to add more developer guide examples
e.g.
```
developer_api_guide/training.py - how to make a tensor subclass trainable
developer_api_guide/fsdp.py - how to make a tensor subclass work with fsdp
developer_api_guide/tensor_parallel.py - how to make a tensor subclass work with tensor parallelism
developer_api_guide/autoquant.py - how to make a tensor subclass work with autoquant
...
```

Test Plan:
python test/tutorials/developer_api_guide/my_dtype_tensor_subclass.py

Reviewers:

Subscribers:

Tasks:

Tags:

Author: jerryzh168

tutorials/developer_api_guide.py renamed to tutorials/developer_api_guide/my_dtype_tensor_subclass.py

@@ -1,9 +1,11 @@
-# Following is a example for a simple dtype implemented with tensor subclass
-# it shows
-#    * the basic structure of a new dtype tensor subclass (__new__, __init__, __tensor_flatten__, __tensor_unflatten__)
-#    * two types of dispatch that people can overwrite (__torch_function__, __torch_dispatch__)
-#    * how to abstract away packing format with layout
-#    * how the tensor subclass composes with torch.compile to get speedup
+"""
+Following is a example for a simple dtype implemented with tensor subclass
+it shows
+    * the basic structure of a new dtype tensor subclass (__new__, __init__, __tensor_flatten__, __tensor_unflatten__)
+    * two types of dispatch that people can overwrite (__torch_function__, __torch_dispatch__)
+    * how to abstract away packing format with layout
+    * how the tensor subclass composes with torch.compile to get speedup
+"""
 
 
 import functools