pytorch/test/cpp/api
Sean McGovern f332017294 C++ API handle optimizer defaults (#161825)
Fixes #141884

This fixes the issue for all optimizers and parameter options.
A member function `overwrite_from` is added to the optimizer base class. Each optimizer then implements this function for comparing their accepted parameters to defaults. A SFINAE approach to handle the different optimizer parameters generically (in optimizer.h only) was evaluated, but I think this is easier to review and maintain.

This mirrors the Python API up to one edge case. An example of the edge case is provided below.

Python can distinguish between 1) Key not present in dict = "not specified"  and 2) Key present in dict = "explicitly set". The C++ implementation cannot.
The issue hinges on whether or not to track if a particular parameter was set by the user explicitly or not (discrepancy in the case when the constructor default is explicitly passed in).

To track this seems like it will take more intervention than would be worth it (modify TORCH_ARG to keep track, use std::optional for the parameter types, use bitset tracking) and was not pursued in the current PR. I'm happy to alter the design if appropriate.

### Example of edge case hinging on CONSTRUCTOR DEFAULTS vs OPTIMIZER DEFAULTS

1. CONSTRUCTOR DEFAULTS:
   These are the values you get when calling AdamOptions()
   AdamOptions().lr() = 0.001
   AdamOptions().weight_decay() = 0
   AdamOptions().eps() = 1e-08

2. OPTIMIZER DEFAULTS:
   These are the values the user chose when creating the optimizer
   User's optimizer defaults:
   optimizer.lr() = 0.005
   optimizer.weight_decay() = 0.1
   optimizer.eps() = 1e-07

3. THE PROBLEM SCENARIO:
   User wants to add a parameter group with explicit weight_decay=0.0
   User sets: weight_decay(0)

4. THE CONFUSION:
   Constructor default weight_decay: 0
   User's explicit weight_decay:     0
   Are they equal? YES

   Since they're equal, our overwrite_from() logic thinks:
   "User didn't set weight_decay explicitly, use optimizer default"

5. CURRENT BEHAVIOR:
   Final weight_decay: 0.1
   User expected:      0
   Match?  NO

=== KEY INSIGHT ===
Constructor defaults are built into the C++ class definition.
Optimizer defaults are chosen by the user at runtime. We want to respect the user intention.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/161825
Approved by: https://github.com/janeyx99
2025-10-08 16:40:45 +00:00
..
any.cpp
autograd.cpp
CMakeLists.txt
dataloader.cpp
dispatch.cpp
enum.cpp
expanding-array.cpp
fft.cpp
functional.cpp
grad_mode.cpp
inference_mode.cpp
init_baseline.h
init_baseline.py
init.cpp
integration.cpp
ivalue.cpp
jit.cpp
memory.cpp
meta_tensor.cpp
misc.cpp
module.cpp
moduledict.cpp
modulelist.cpp
modules.cpp
namespace.cpp
nested_int.cpp
nested.cpp
nn_utils.cpp
operations.cpp
optim_baseline.h
optim_baseline.py
optim.cpp C++ API handle optimizer defaults (#161825) 2025-10-08 16:40:45 +00:00
ordered_dict.cpp
parallel_benchmark.cpp
parallel.cpp
parameterdict.cpp
parameterlist.cpp
README.md
rnn.cpp
sequential.cpp
serialize.cpp
special.cpp
static.cpp
support.cpp
support.h
tensor_cuda.cpp
tensor_flatten.cpp
tensor_indexing.cpp
tensor_options_cuda.cpp
tensor_options.cpp
tensor.cpp
torch_include.cpp
transformer.cpp

C++ Frontend Tests

In this folder live the tests for PyTorch's C++ Frontend. They use the GoogleTest test framework.

CUDA Tests

To make a test runnable only on platforms with CUDA, you should suffix your test with _CUDA, e.g.

TEST(MyTestSuite, MyTestCase_CUDA) { }

To make it runnable only on platforms with at least two CUDA machines, suffix it with _MultiCUDA instead of _CUDA, e.g.

TEST(MyTestSuite, MyTestCase_MultiCUDA) { }

There is logic in main.cpp that detects the availability and number of CUDA devices and supplies the appropriate negative filters to GoogleTest.

Integration Tests

Integration tests use the MNIST dataset. You must download it by running the following command from the PyTorch root folder:

$ python tools/download_mnist.py -d test/cpp/api/mnist

The required paths will be referenced as test/cpp/api/mnist/... in the test code, so you must run the integration tests from the PyTorch root folder.