Fix nbdev export system across all 20 modules

PROBLEM:
- nbdev requires #| export directive on EACH cell to export when using # %% markers
- Cell markers inside class definitions split classes across multiple cells
- Only partial classes were being exported to tinytorch package
- Missing matmul, arithmetic operations, and activation classes in exports

SOLUTION:
1. Removed # %% cell markers INSIDE class definitions (kept classes as single units)
2. Added #| export to imports cell at top of each module
3. Added #| export before each exportable class definition in all 20 modules
4. Added __call__ method to Sigmoid for functional usage
5. Fixed numpy import (moved to module level from __init__)

MODULES FIXED:
- 01_tensor: Tensor class with all operations (matmul, arithmetic, shape ops)
- 02_activations: Sigmoid, ReLU, Tanh, GELU, Softmax classes
- 03_layers: Linear, Dropout classes
- 04_losses: MSELoss, CrossEntropyLoss, BinaryCrossEntropyLoss classes
- 05_autograd: Function, AddBackward, MulBackward, MatmulBackward, SumBackward
- 06_optimizers: Optimizer, SGD, Adam, AdamW classes
- 07_training: CosineSchedule, Trainer classes
- 08_dataloader: Dataset, TensorDataset, DataLoader classes
- 09_spatial: Conv2d, MaxPool2d, AvgPool2d, SimpleCNN classes
- 10-20: All exportable classes in remaining modules

TESTING:
- Test functions use 'if __name__ == "__main__"' guards
- Tests run in notebooks but NOT on import
- Rosenblatt Perceptron milestone working perfectly

RESULT:
✅ All 20 modules export correctly
✅ Perceptron (1957) milestone functional
✅ Clean separation: development (modules/source) vs package (tinytorch)

modules/source/17_quantization/quantization_dev.py

@@ -778,6 +778,7 @@ Regular Linear Layer:           QuantizedLinear Layer:
 """
 
 # %% nbgrader={"grade": false, "grade_id": "quantized_linear", "solution": true}
+#| export
 class QuantizedLinear:
     """Quantized version of Linear layer using INT8 arithmetic."""