Updating documentation for cmake dtype selective build (#12112)

### Summary
Updating documentation for cmake dtype selective build with model API.
Table at
`selective_build_model_doc/docs/source/kernel-library-selective-build.md`
now looks like:


![image](https://github.com/user-attachments/assets/68b7c8fe-dfc5-4ae4-9756-5599680357bf)


Fixes #12065

cc @mergennachin @byjlw

Author: BujSet

docs/source/kernel-library-selective-build.md

@@ -36,17 +36,20 @@ The basic flow looks like this:
 
 ## APIs
 
-We expose a CMake macro `[gen_selected_ops](https://github.com/pytorch/executorch/blob/main/tools/cmake/Codegen.cmake#L12)`, to allow users specifying op info:
+We expose a CMake macro [gen_selected_ops](https://github.com/pytorch/executorch/blob/main/tools/cmake/Codegen.cmake#L12), to allow users specifying op info:
 
 ```
 gen_selected_ops(
-  LIB_NAME         # the name of the selective build operator library to be generated
-  OPS_SCHEMA_YAML  # path to a yaml file containing operators to be selected
-  ROOT_OPS         # comma separated operator names to be selected
-  INCLUDE_ALL_OPS  # boolean flag to include all operators
+  LIB_NAME              # the name of the selective build operator library to be generated
+  OPS_SCHEMA_YAML       # path to a yaml file containing operators to be selected
+  ROOT_OPS              # comma separated operator names to be selected
+  INCLUDE_ALL_OPS       # boolean flag to include all operators
+  OPS_FROM_MODEL        # path to a pte file of model to select operators from
+  DTYPE_SELECTIVE_BUILD # boolean flag to enable dtye selection
 )
 ```
 
+The macro makes a call to gen_oplist.py, which requires a [distinct selection](https://github.com/BujSet/executorch/blob/main/codegen/tools/gen_oplist.py#L222-L228) of API choice. `OPS_SCHEMA_YAML`, `ROOT_OPS`, `INCLUDE_ALL_OPS`, and `OPS_FROM_MODEL` are mutually exclusive options, and should not be used in conjunction. 
 
 ### Select all ops
 
@@ -62,31 +65,29 @@ Context: each kernel library is designed to have a yaml file associated with it.
 
 This API lets users pass in a list of operator names. Note that this API can be combined with the API above and we will create a allowlist from the union of both API inputs.
 
+### Select ops from model
 
-## Example Walkthrough
+This API lets users pass in a pte file of an exported model. When used, the pte file will be parsed to generate a yaml file that enumerates the operators and dtypes used in the model. 
 
-In CMakeLists.txt we have the following logic:
-```cmake
-set(_kernel_lib)
-if(SELECT_ALL_OPS)
-  gen_selected_ops("" "" "${SELECT_ALL_OPS}")
-elseif(SELECT_OPS_LIST)
-  gen_selected_ops("" "${SELECT_OPS_LIST}" "")
-elseif(SELECT_OPS_YAML)
- set(_custom_ops_yaml ${EXECUTORCH_ROOT}/examples/portable/custom_ops/custom_ops.yaml)
-  gen_selected_ops("${_custom_ops_yaml}" "" "")
-endif()
-```
-Then when calling CMake, we can do:
+### Dtype Selective Build
 
-```
-cmake -D… -DSELECT_OPS_LIST="aten::add.out,aten::mm.out”
-```
+Beyond pruning the binary to remove unused operators, the binary size can further reduced by removing unused dtypes. For example, if your model only uses floats for the `add` operator, then including variants of the `add` operators for `doubles` and `ints` is unnecessary. The flag `DTYPE_SELECTIVE_BUILD` can be set to `ON` to support this additional optimization. Currently, dtype selective build is only supported with the model API described above. Once enabled, a header file that specifies only the operators and dtypes used by the model is created and linked against a rebuild of the `portable_kernels` lib. This feature is only supported for the portable kernels library; it's not supported for optimized, quantized or custom kernel libraries.
 
-Or
+## Example Walkthrough
 
-```
-cmake -D… -DSELECT_OPS_YAML=ON
-```
+In [CMakeLists.txt](https://github.com/BujSet/executorch/blob/main/examples/selective_build/CMakeLists.txt#L48-L72), we have the following cmake config options:
+
+1. `EXECUTORCH_SELECT_OPS_YAML`
+2. `EXECUTORCH_SELECT_OPS_LIST`
+3. `EXECUTORCH_SELECT_ALL_OPS`
+4. `EXECUTORCH_SELECT_OPS_FROM_MODEL`
+5. `EXECUTORCH_DTYPE_SELECTIVE_BUILD`
+
+These options allow a user to tailor the cmake build process to utilize the different APIs, and results in different invocations on the `gen_selected_ops` [function](https://github.com/BujSet/executorch/blob/main/examples/selective_build/CMakeLists.txt#L110-L123). The following table describes some examples of how the invocation changes when these configs are set:
 
-To select from either an operator name list or a schema yaml from kernel library.
+| Example cmake Call | Resultant `gen_selected_ops` Invocation |
+| :----: | :---:| 
+|<code><br>  cmake -D… -DSELECT_OPS_LIST="aten::add.out,aten::mm.out" <br></code> | <code><br>  gen_selected_ops("" "${SELECT_OPS_LIST}" "" "" "") <br></code> |
+|<code><br> cmake -D… -DSELECT_OPS_YAML=ON <br></code> | <code><br>  set(_custom_ops_yaml ${EXECUTORCH_ROOT}/examples/portable/custom_ops/custom_ops.yaml) <br> gen_selected_ops("${_custom_ops_yaml}" "" "") <br></code> |
+|<code><br> cmake -D… -DEXECUTORCH_SELECT_OPS_FROM_MODEL="model.pte.out" <br></code> | <code><br> gen_selected_ops("" "" "" "${_model_path}" "") <br></code> |
+|<code><br> cmake -D… -DEXECUTORCH_SELECT_OPS_FROM_MODEL="model.pte.out" -DEXECUTORCH_DTYPE_SELECTIVE_BUILD=ON<br></code> | <code><br> gen_selected_ops("" "" "" "${_model_path}" "ON") <br></code> |

examples/selective_build/README.md

@@ -12,12 +12,12 @@ cd executorch
 bash examples/selective_build/test_selective_build.sh cmake
 ```
 
-Check out `CMakeLists.txt` for demo of 3 selective build APIs:
+Check out `CMakeLists.txt` for demo of selective build APIs:
 1. `SELECT_ALL_OPS`: Select all ops from the dependency kernel libraries, register all of them into ExecuTorch runtime.
 2. `SELECT_OPS_LIST`: Only select operators from a list.
 3. `SELECT_OPS_YAML`: Only select operators from a yaml file.
+4. `SELECT_OPS_FROM_MODEL`: Only select operators from a from an exported model pte.
+5. `DTYPE_SELECTIVE_BUILD`: Enable rebuild of `portable_kernels` to use dtype selection. Currently only supported for `SELECTED_OPS_FROM_MODEL` API and `portable_kernels` lib.
 
 Other configs:
 - `MAX_KERNEL_NUM=N`: Only allocate memory for N operators.
-
-We have one more API incoming: only select from an exported model file (.pte).