Fix warning about uninitialized member (#16736)
#16506 Cause almost every translation units on linux complaint
```
[1175/1235] Building CXX object CMakeFiles/onnxruntime_test_all.dir/home/guangyunhan/onnxruntime/orttraining/orttraining/test/training_ops/cuda/softmax_test.cc.o
In file included from /home/guangyunhan/onnxruntime/include/onnxruntime/core/framework/float16.h:18,
from /home/guangyunhan/onnxruntime/include/onnxruntime/core/framework/data_types.h:17,
from /home/guangyunhan/onnxruntime/include/onnxruntime/core/framework/tensor.h:17,
from /home/guangyunhan/onnxruntime/onnxruntime/test/common/tensor_op_test_utils.h:16,
from /home/guangyunhan/onnxruntime/onnxruntime/test/providers/compare_provider_test_utils.h:7,
from /home/guangyunhan/onnxruntime/orttraining/orttraining/test/training_ops/cuda/softmax_test.cc:4:
/home/guangyunhan/onnxruntime/include/onnxruntime/core/session/onnxruntime_float16.h: In instantiation of ‘static constexpr uint16_t onnxruntime_float16::Float16Impl<Derived>::ToUint16Impl(float) [with Derived = onnxruntime::MLFloat16; uint16_t = short unsigned int]’:
/home/guangyunhan/onnxruntime/include/onnxruntime/core/framework/float16.h:42:66: required from here
/home/guangyunhan/onnxruntime/include/onnxruntime/core/session/onnxruntime_float16.h:241:7: note: ‘union onnxruntime_float16::detail::float32_bits’ has no user-provided default constructor
241 | union float32_bits {
| ^~~~~~~~~~~~
/home/guangyunhan/onnxruntime/include/onnxruntime/core/session/onnxruntime_float16.h:242:16: note: and the implicitly-defined constructor does not initialize ‘unsigned int onnxruntime_float16::detail::float32_bits::u’
242 | unsigned int u;
| ^
```
This PR shut the compiler up.
