Change trust_remote_code default to False for security reason (#787)

## What does this PR do?

**Type of change:** ? <!-- Use one of the following: Bug fix, new
feature, new example, new tests, documentation. --> Bug fix

**Overview:** ?

Change `trust_remote_code` default to `False` for security reason

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Bug Fixes**
* Updated model loader security settings: remote code is no longer
trusted by default when loading model configurations. Users requiring
remote code execution must now explicitly enable this option.

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settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Chenhan Yu <chenhany@nvidia.com>

Author: ChenhanYu

modelopt/torch/export/plugins/megatron_importer.py

@@ -75,7 +75,7 @@ def __init__(
         workspace_dir: str | None = None,
         dtype=torch.bfloat16,
         dequantize: bool = True,
-        trust_remote_code: bool = True,
+        trust_remote_code: bool = False,
         verbose: bool = False,
         moe_router_dtype: torch.dtype | None = None,
     ):