refactor: adopt _src pattern and migrate core components

- Restructured the repository to separate internal logic (`pina/_src`) from
  the public API.
- Migrated central components (Graph, LabelTensor, Operator, Trainer) into
  `pina/_src/core/` for better organization.
- Updated all subpackage `__init__.py` files to expose a clean, flat
  import surface for users.
- Standardized module docstrings across Solvers, Models, Equations,
  Domains, and Callbacks to improve documentation quality.
- Optimized internal dictionary handling in tensor storage functions.

Author: adendek

pina/__init__.py

@@ -1,18 +1,24 @@
-"""Module for the Pina library."""
+"""
+PINA: Physics-Informed Neural Analysis.
+
+A specialized framework for Scientific Machine Learning (SciML), providing
+tools for Physics-Informed Neural Networks (PINNs), Neural Operators,
+and data-driven physical modeling.
+"""
 
 __all__ = [
-    "Trainer",
     "LabelTensor",
+    "Trainer",
     "Condition",
     "PinaDataModule",
     "Graph",
     "SolverInterface",
     "MultiSolverInterface",
 ]
 
-from .label_tensor import LabelTensor
-from .graph import Graph
-from .solver import SolverInterface, MultiSolverInterface
-from .trainer import Trainer
-from .condition.condition import Condition
-from .data import PinaDataModule
+from pina._src.core.label_tensor import LabelTensor
+from pina._src.core.graph import Graph
+from pina._src.solver.solver import SolverInterface, MultiSolverInterface
+from pina._src.core.trainer import Trainer
+from pina._src.condition.condition import Condition
+from pina._src.data.data_module import PinaDataModule

pina/_src/__init__.py

@@ -1,8 +1,8 @@
 """Module for the Adaptive Functions."""
 
 import torch
-from ..utils import check_consistency
-from .adaptive_function_interface import AdaptiveActivationFunctionInterface
+from pina._src.core.utils import check_consistency
+from pina._src.adaptive_function.adaptive_function_interface import AdaptiveActivationFunctionInterface
 
 
 class AdaptiveReLU(AdaptiveActivationFunctionInterface):

pina/_src/adaptive_function/__init__.py

@@ -2,7 +2,7 @@
 
 from abc import ABCMeta
 import torch
-from ..utils import check_consistency, is_function
+from pina._src.core.utils import check_consistency, is_function
 
 
 class AdaptiveActivationFunctionInterface(torch.nn.Module, metaclass=ABCMeta):

…a/adaptive_function/adaptive_function.py …c/adaptive_function/adaptive_function.pypina/adaptive_function/adaptive_function.py renamed to pina/_src/adaptive_function/adaptive_function.py pina/adaptive_function/adaptive_function.py renamed to pina/_src/adaptive_function/adaptive_function.py

@@ -1,8 +1,8 @@
 """Module for the SwitchOptimizer callback."""
 
 from lightning.pytorch.callbacks import Callback
-from ...optim import TorchOptimizer
-from ...utils import check_consistency
+from pina._src.optim.torch_optimizer import TorchOptimizer
+from pina._src.core.utils import check_consistency
 
 
 class SwitchOptimizer(Callback):

…_function/adaptive_function_interface.py …_function/adaptive_function_interface.pypina/adaptive_function/adaptive_function_interface.py renamed to pina/_src/adaptive_function/adaptive_function_interface.py pina/adaptive_function/adaptive_function_interface.py renamed to pina/_src/adaptive_function/adaptive_function_interface.py

@@ -1,8 +1,8 @@
 """Module for the SwitchScheduler callback."""
 
 from lightning.pytorch.callbacks import Callback
-from ...optim import TorchScheduler
-from ...utils import check_consistency, check_positive_integer
+from pina._src.optim.torch_scheduler import TorchScheduler
+from pina._src.core.utils import check_consistency, check_positive_integer
 
 
 class SwitchScheduler(Callback):