[Bug] All-zero logits from convert_exported_program_to_serialized_trt_engine with use_explicit_typing=False

Bug description

convert_exported_program_to_serialized_trt_engine produces a TRT engine with all-zero logits when use_explicit_typing=False and enabled_precisions={torch.float16}. The same exported program converts correctly with use_explicit_typing=True.

No error or warning is emitted — the engine silently produces incorrect results.

To reproduce

import torch
import torch_tensorrt
from transformers import AutoModelForCausalLM, StaticCache

model = AutoModelForCausalLM.from_pretrained("Qwen/Qwen3-0.6B", dtype=torch.float16, device_map="cuda")
model.eval()

# Export with stateless KV cache wrapper
# (Standard torch.export flow — model has fp16 weights, fp32 I/O)
# ... export model to ExportedProgram ...

# This produces all-zero logits:
engine_bad = torch_tensorrt.dynamo.convert_exported_program_to_serialized_trt_engine(
    exported_program,
    inputs=example_inputs,
    use_explicit_typing=False,
    enabled_precisions={torch.float16},
    min_block_size=1,
    truncate_double=True,
)

# This produces correct logits:
engine_good = torch_tensorrt.dynamo.convert_exported_program_to_serialized_trt_engine(
    exported_program,
    inputs=example_inputs,
    use_explicit_typing=True,
    min_block_size=1,
    truncate_double=True,
)

Observed behavior

• use_explicit_typing=False: Engine builds successfully (~1139 MB), but inference produces logits where every value is exactly 0.0
• use_explicit_typing=True: Engine builds successfully (~1139 MB) with correct non-zero logits

Both engines are the same size, suggesting TRT builds a valid-looking engine in both cases, but the use_explicit_typing=False path silently corrupts computation — likely due to incorrect type casting or layer fusion.

Expected behavior

Both paths should produce correct (non-zero) logits, or use_explicit_typing=False should raise an error/warning if it cannot handle the exported program correctly.

Environment

• torch: 2.10.0+cu128
• torch_tensorrt: 2.10.0+cu130
• tensorrt: 10.14.1.48
• transformers: 5.2.0
• Model: Qwen/Qwen3-0.6B (fp16, 28 layers, GQA with 16 heads / 2 KV heads)
• GPU: NVIDIA GeForce RTX 4090 (24GB)
• Driver: 590.48.01
• OS: Linux (Ubuntu 24.04)

Start at torch_tensorrt.dynamo.convert_exported_program_to_serialized_trt_engine and reproduce the Qwen/Qwen3-0.6B case with use_explicit_typing=False and enabled_precisions={torch.float16}. Compare it with the explicit-typing path and inspect the generated engine's logits. Done means the non-explicit path produces correct non-zero logits or reports that the configuration is unsupported.