feat: support non-Triton platforms (aarch64/Jetson) — conditional torch.compile

torch.compile requires Triton which is unavailable on aarch64 (NVIDIA
Jetson, ARM servers). This makes compilation conditional:

- nanochat/optim.py: Replace hardcoded @torch.compile decorators with
  _compile_decorator that becomes a no-op when Triton is not installed
- scripts/base_train.py: Wrap torch.compile(model) in try/except
- nanochat/common.py: Add Orin to peak FLOPS table (5.3 TFLOPS BF16)

Tested end-to-end on NVIDIA Jetson Orin NX 8GB (Compute 8.7, CUDA 12.6,
PyTorch 2.8.0) — pretrain and SFT both run successfully in eager mode.
No changes to behavior on platforms where Triton is available.

nanochat/common.py

@@ -264,6 +264,8 @@ def get_peak_flops(device_name: str) -> float:
         (["5090"], 209.5e12),
         (["4090"], 165.2e12),
         (["3090"], 71e12),
+        # NVIDIA Jetson (unified memory, iGPU)
+        (["orin"], 5.3e12),  # Jetson Orin NX 8GB: ~5.3 TFLOPS BF16 (8 SMs @ 1.0GHz)
     )
     for patterns, flops in _PEAK_FLOPS_TABLE:
         if all(p in name for p in patterns):

nanochat/optim.py

@@ -11,13 +11,21 @@ import torch
 import torch.distributed as dist
 from torch import Tensor
 
+# Check if torch.compile works (requires Triton on CUDA)
+_can_compile = True
+try:
+    import triton  # noqa: F401
+except ImportError:
+    _can_compile = False
+_compile_decorator = torch.compile(dynamic=False, fullgraph=True) if _can_compile else (lambda fn: fn)
+
 # -----------------------------------------------------------------------------
 """
 Good old AdamW optimizer, fused kernel.
 https://arxiv.org/abs/1711.05101
 """
 
-@torch.compile(dynamic=False, fullgraph=True)
+@_compile_decorator
 def adamw_step_fused(
     p: Tensor,              # (32768, 768) - parameter tensor
     grad: Tensor,           # (32768, 768) - gradient, same shape as p
@@ -87,7 +95,7 @@ polar_express_coeffs = [
     (2.3465413258596377, -1.7097828382687081, 0.42323551169305323),
 ]
 
-@torch.compile(dynamic=False, fullgraph=True)
+@_compile_decorator
 def muon_step_fused(
     stacked_grads: Tensor,          # (12, 768, 3072) - stacked gradients
     stacked_params: Tensor,         # (12, 768, 3072) - stacked parameters

scripts/base_train.py

@@ -244,7 +244,17 @@ def disable_fp8(model):
 # Compile the model
 
 orig_model = model # original, uncompiled model, for saving raw model state_dict and for inference/evaluation (because the shapes may change shape)
-model = torch.compile(model, dynamic=False) # the inputs to model will never change shape so dynamic=False is safe
+# torch.compile requires Triton, which is not available on all platforms (e.g. Jetson aarch64)
+_can_compile = True
+try:
+    import triton  # noqa: F401
+except ImportError:
+    _can_compile = False
+if _can_compile and device_type == "cuda":
+    model = torch.compile(model, dynamic=False) # the inputs to model will never change shape so dynamic=False is safe
+    print0("✓ torch.compile enabled")
+else:
+    print0("⚠ torch.compile disabled (Triton not available or non-CUDA device), running in eager mode")
 
 # -----------------------------------------------------------------------------
 # Scaling laws and muP extrapolations to determine the optimal training horizon, batch size, learning rates, weight decay.