This change addresses confusing error messages users encounter when using the ONNX exporter with default settings. Previously, `fallback=True` was the default, which would attempt to fall back to the TorchScript exporter when the dynamo path failed, leading to mixed error messages that obscured the actual issues.

## Problem

When `fallback=True` by default:
- Users get confusing error messages mixing dynamo and TorchScript export failures
- Error messages tell users to provide the `f` argument unnecessarily
- Dynamo error messages get flushed with TorchScript errors when both paths fail
- Users expecting the dynamo path get unexpected fallback behavior

## Solution

Changed the default from `fallback=True` to `fallback=False` in both:
- `torch.onnx.export()` function
- `torch.onnx._internal.exporter._compat.export_compat()` function

## Impact

**Before:**
```python
# Would fallback to TorchScript on dynamo failure, causing mixed error messages
torch.onnx.export(model, args)
```

**After:**
```python
# Clean dynamo-only errors by default
torch.onnx.export(model, args)

# Advanced users can still opt-in to fallback behavior
torch.onnx.export(model, args, fallback=True)
```

Fixes pytorch#162697

Pull Request resolved: pytorch#162726
Approved by: https://github.com/titaiwangms, https://github.com/xadupre

This change addresses confusing error messages users encounter when using the ONNX exporter with default settings. Previously, `fallback=True` was the default, which would attempt to fall back to the TorchScript exporter when the dynamo path failed, leading to mixed error messages that obscured the actual issues.

## Problem

When `fallback=True` by default:
- Users get confusing error messages mixing dynamo and TorchScript export failures
- Error messages tell users to provide the `f` argument unnecessarily
- Dynamo error messages get flushed with TorchScript errors when both paths fail
- Users expecting the dynamo path get unexpected fallback behavior

## Solution

Changed the default from `fallback=True` to `fallback=False` in both:
- `torch.onnx.export()` function
- `torch.onnx._internal.exporter._compat.export_compat()` function

## Impact

**Before:**
```python
# Would fallback to TorchScript on dynamo failure, causing mixed error messages
torch.onnx.export(model, args)
```

**After:**
```python
# Clean dynamo-only errors by default
torch.onnx.export(model, args)

# Advanced users can still opt-in to fallback behavior
torch.onnx.export(model, args, fallback=True)
```

Fixes pytorch#162697

Pull Request resolved: pytorch#162726
Approved by: https://github.com/titaiwangms, https://github.com/xadupre

This change addresses confusing error messages users encounter when using the ONNX exporter with default settings. Previously, `fallback=True` was the default, which would attempt to fall back to the TorchScript exporter when the dynamo path failed, leading to mixed error messages that obscured the actual issues.

## Problem

When `fallback=True` by default:
- Users get confusing error messages mixing dynamo and TorchScript export failures
- Error messages tell users to provide the `f` argument unnecessarily
- Dynamo error messages get flushed with TorchScript errors when both paths fail
- Users expecting the dynamo path get unexpected fallback behavior

## Solution

Changed the default from `fallback=True` to `fallback=False` in both:
- `torch.onnx.export()` function
- `torch.onnx._internal.exporter._compat.export_compat()` function

## Impact

**Before:**
```python
# Would fallback to TorchScript on dynamo failure, causing mixed error messages
torch.onnx.export(model, args)
```

**After:**
```python
# Clean dynamo-only errors by default
torch.onnx.export(model, args)

# Advanced users can still opt-in to fallback behavior
torch.onnx.export(model, args, fallback=True)
```

Fixes pytorch#162697

Pull Request resolved: pytorch#162726
Approved by: https://github.com/titaiwangms, https://github.com/xadupre

This change addresses confusing error messages users encounter when using the ONNX exporter with default settings. Previously, `fallback=True` was the default, which would attempt to fall back to the TorchScript exporter when the dynamo path failed, leading to mixed error messages that obscured the actual issues.

## Problem

When `fallback=True` by default:
- Users get confusing error messages mixing dynamo and TorchScript export failures
- Error messages tell users to provide the `f` argument unnecessarily
- Dynamo error messages get flushed with TorchScript errors when both paths fail
- Users expecting the dynamo path get unexpected fallback behavior

## Solution

Changed the default from `fallback=True` to `fallback=False` in both:
- `torch.onnx.export()` function
- `torch.onnx._internal.exporter._compat.export_compat()` function

## Impact

**Before:**
```python
# Would fallback to TorchScript on dynamo failure, causing mixed error messages
torch.onnx.export(model, args)
```

**After:**
```python
# Clean dynamo-only errors by default
torch.onnx.export(model, args)

# Advanced users can still opt-in to fallback behavior
torch.onnx.export(model, args, fallback=True)
```

Fixes pytorch#162697

Pull Request resolved: pytorch#162726
Approved by: https://github.com/titaiwangms, https://github.com/xadupre