Restructure: Separate developer source (src/) from learner notebooks (modules/)

Major directory restructure to support both developer and learner workflows:

Structure Changes:
- NEW: src/ directory for Python source files (version controlled)
  - Files renamed: tensor.py → 01_tensor.py (matches directory naming)
  - All 20 modules moved from modules/ to src/
- CHANGED: modules/ now holds generated notebooks (gitignored)
  - Generated from src/*.py using jupytext
  - Learners work in notebooks, developers work in Python source
- UNCHANGED: tinytorch/ package (still auto-generated from notebooks)

Workflow: src/*.py → modules/*.ipynb → tinytorch/*.py

Command Updates:
- Updated export command to read from src/ and generate to modules/
- Export flow: discovers modules in src/, converts to notebooks in modules/, exports to tinytorch/
- All 20 modules tested and working

Configuration:
- Updated .gitignore to ignore modules/ directory
- Updated README.md with new three-layer architecture explanation
- Updated export.py source mappings and paths

Benefits:
- Clean separation: developers edit Python, learners use notebooks
- Better version control: only Python source committed, notebooks generated
- Flexible learning: can work in notebooks OR Python source
- Maintains backward compatibility: tinytorch package unchanged

Tested:
- Single module export: tito export 01_tensor ✅
- All modules export: tito export --all ✅
- Package imports: from tinytorch.core.tensor import Tensor ✅
- 20/20 modules successfully converted and exported

tinytorch/data/loader.py

@@ -5,33 +5,35 @@
 # ║  This file is AUTOMATICALLY GENERATED from source modules.                   ║
 # ║  ANY CHANGES MADE HERE WILL BE LOST when modules are re-exported!            ║
 # ║                                                                               ║
-# ║  ✅ TO EDIT: modules/XX_loader/loader.py                            ║
+# ║  ✅ TO EDIT: src/XX_loader/XX_loader.py                             ║
 # ║  ✅ TO EXPORT: Run 'tito module complete <module_name>'                      ║
 # ║                                                                               ║
 # ║  🛡️ STUDENT PROTECTION: This file contains optimized implementations.        ║
 # ║     Editing it directly may break module functionality and training.         ║
 # ║                                                                               ║
-# ║  🎓 LEARNING TIP: Work in modules/ - that's where real development    ║
-# ║     happens! The tinytorch/ directory is just the compiled output.           ║
+# ║  🎓 LEARNING TIP: Work in src/ (developers) or modules/ (learners)    ║
+# ║     The tinytorch/ directory is generated code - edit source files instead!  ║
 # ╚═══════════════════════════════════════════════════════════════════════════════╝
 # %% auto 0
 __all__ = ['Dataset', 'TensorDataset', 'DataLoader']
 
-# %% ../../modules/source/08_dataloader/dataloader_dev.ipynb 0
+# %% ../../modules/08_dataloader/08_dataloader.ipynb 0
 #| default_exp data.loader
 #| export
 
-# %% ../../modules/source/08_dataloader/dataloader_dev.ipynb 2
+# %% ../../modules/08_dataloader/08_dataloader.ipynb 2
 # Essential imports for data loading
 import numpy as np
 import random
+import time
+import sys
 from typing import Iterator, Tuple, List, Optional, Union
 from abc import ABC, abstractmethod
 
 # Import real Tensor class from tinytorch package
 from ..core.tensor import Tensor
 
-# %% ../../modules/source/08_dataloader/dataloader_dev.ipynb 4
+# %% ../../modules/08_dataloader/08_dataloader.ipynb 4
 class Dataset(ABC):
     """
     Abstract base class for all datasets.
@@ -84,7 +86,7 @@ class Dataset(ABC):
         pass
     ### END SOLUTION
 
-# %% ../../modules/source/08_dataloader/dataloader_dev.ipynb 7
+# %% ../../modules/08_dataloader/08_dataloader.ipynb 7
 class TensorDataset(Dataset):
     """
     Dataset wrapping tensors for supervised learning.
@@ -161,7 +163,7 @@ class TensorDataset(Dataset):
         return tuple(Tensor(tensor.data[idx]) for tensor in self.tensors)
         ### END SOLUTION
 
-# %% ../../modules/source/08_dataloader/dataloader_dev.ipynb 10
+# %% ../../modules/08_dataloader/08_dataloader.ipynb 10
 class DataLoader:
     """
     Data loader with batching and shuffling support.