sg_trainer loads checkpoint before metric_to_watch initialization

[JackW4529]

🐛 Describe the bug

Bug Description

In sg_trainer, the checkpoint is loaded before metric_to_watch is initialized from training_params.
This causes _best_ckpt_metrics to behave incorrectly, because self.metric_to_watch is None during the checkpoint loading process.

Basically, the trainer is doing:

self._load_checkpoint_to_model()  # checkpoint loads first
self.metric_to_watch = self.training_params.metric_to_watch  # metric_to_watch is set too late


### Versions

PyTorch version: 2.5.0+cu124
Is debug build: False
CUDA used to build PyTorch: 12.4
ROCM used to build PyTorch: N/A

OS: Ubuntu 22.04.4 LTS (x86_64)
GCC version: (Ubuntu 11.4.0-1ubuntu1~22.04.2) 11.4.0
Clang version: 14.0.0-1ubuntu1.1
CMake version: version 3.31.6
Libc version: glibc-2.35

Python version: 3.12.11 (main, Jun  4 2025, 08:56:18) [GCC 11.4.0] (64-bit runtime)
Python platform: Linux-6.6.97+-x86_64-with-glibc2.35
Is CUDA available: True
CUDA runtime version: 12.5.82
CUDA_MODULE_LOADING set to: LAZY
GPU models and configuration: GPU 0: Tesla T4
Nvidia driver version: 550.54.15
cuDNN version: Probably one of the following:
/usr/lib/x86_64-linux-gnu/libcudnn.so.9.2.1
/usr/lib/x86_64-linux-gnu/libcudnn_adv.so.9.2.1
/usr/lib/x86_64-linux-gnu/libcudnn_cnn.so.9.2.1
/usr/lib/x86_64-linux-gnu/libcudnn_engines_precompiled.so.9.2.1
/usr/lib/x86_64-linux-gnu/libcudnn_engines_runtime_compiled.so.9.2.1
/usr/lib/x86_64-linux-gnu/libcudnn_graph.so.9.2.1
/usr/lib/x86_64-linux-gnu/libcudnn_heuristic.so.9.2.1
/usr/lib/x86_64-linux-gnu/libcudnn_ops.so.9.2.1
Is XPU available: False
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

CPU:
Architecture:                            x86_64
CPU op-mode(s):                          32-bit, 64-bit
Address sizes:                           46 bits physical, 48 bits virtual
Byte Order:                              Little Endian
CPU(s):                                  2
On-line CPU(s) list:                     0,1
Vendor ID:                               GenuineIntel
Model name:                              Intel(R) Xeon(R) CPU @ 2.20GHz
CPU family:                              6
Model:                                   79
Thread(s) per core:                      2
Core(s) per socket:                      1
Socket(s):                               1
Stepping:                                0
BogoMIPS:                                4399.99
Flags:                                   fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm constant_tsc rep_good nopl xtopology nonstop_tsc cpuid tsc_known_freq pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch ssbd ibrs ibpb stibp fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm rdseed adx smap xsaveopt arat md_clear arch_capabilities
Hypervisor vendor:                       KVM
Virtualization type:                     full
L1d cache:                               32 KiB (1 instance)
L1i cache:                               32 KiB (1 instance)
L2 cache:                                256 KiB (1 instance)
L3 cache:                                55 MiB (1 instance)
NUMA node(s):                            1
NUMA node0 CPU(s):                       0,1
Vulnerability Gather data sampling:      Not affected
Vulnerability Indirect target selection: Vulnerable
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Mitigation; PTE Inversion
Vulnerability Mds:                       Vulnerable; SMT Host state unknown
Vulnerability Meltdown:                  Vulnerable
Vulnerability Mmio stale data:           Vulnerable
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Vulnerable
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Vulnerable
Vulnerability Spectre v1:                Vulnerable: __user pointer sanitization and usercopy barriers only; no swapgs barriers
Vulnerability Spectre v2:                Vulnerable; IBPB: disabled; STIBP: disabled; PBRSB-eIBRS: Not affected; BHI: Vulnerable
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Vulnerable

Versions of relevant libraries:
[pip3] intel-cmplr-lib-ur==2025.2.1
[pip3] intel-openmp==2025.2.1
[pip3] mkl==2025.2.0
[pip3] numpy==2.0.2
[pip3] nvidia-cublas-cu12==12.4.5.8
[pip3] nvidia-cuda-cupti-cu12==12.4.127
[pip3] nvidia-cuda-nvrtc-cu12==12.4.127
[pip3] nvidia-cuda-runtime-cu12==12.4.127
[pip3] nvidia-cudnn-cu12==9.1.0.70
[pip3] nvidia-cufft-cu12==11.2.1.3
[pip3] nvidia-curand-cu12==10.3.5.147
[pip3] nvidia-cusolver-cu12==11.6.1.9
[pip3] nvidia-cusparse-cu12==12.3.1.170
[pip3] nvidia-cusparselt-cu12==0.7.1
[pip3] nvidia-nccl-cu12==2.21.5
[pip3] nvidia-nvjitlink-cu12==12.4.127
[pip3] nvidia-nvtx-cu12==12.4.127
[pip3] nvtx==0.2.13
[pip3] onnx==1.15.0
[pip3] onnxruntime==1.22.1
[pip3] onnxsim==0.4.36
[pip3] optree==0.17.0
[pip3] pynvjitlink-cu12==0.7.0
[pip3] tbb==2022.2.0
[pip3] tcmlib==1.4.0
[pip3] torch==2.5.0
[pip3] torchao==0.10.0
[pip3] torchdata==0.11.0
[pip3] torchmetrics==0.8.0
[pip3] torchsummary==1.5.1
[pip3] torchtune==0.6.1
[pip3] torchvision==0.20.0
[pip3] triton==3.1.0
[pip3] umf==0.11.0
[conda] Could not collect