Refactor error messages and function signatures across multiple notebooks for clarity and consistency

- Updated error messages in Gauss method and numerical methods to use variables for better readability.
- Added return type hints to function signatures in various notebooks to improve code documentation.
- Corrected minor grammatical issues in docstrings for better clarity.
- Adjusted print statements and list concatenations for improved output formatting.
- Enhanced plotting functions to ensure consistent figure handling.

M2/Deep Learning/TP4 - Récurrents/TP4 - Starter.ipynb

@@ -177,7 +177,7 @@
     "import _pickle as pickle\n",
     "\n",
     "train_size = 0.8\n",
-    "train_index = int(round(len(sequences) * train_size))\n",
+    "train_index = round(len(sequences) * train_size)\n",
     "X_train = X[:train_index, :, :]\n",
     "y_train = y[:train_index, :]\n",
     "\n",
@@ -471,7 +471,7 @@
    },
    "outputs": [],
    "source": [
-    "def save_model(model, name):\n",
+    "def save_model(model, name) -> None:\n",
     "    \"\"\"Save a Keras model to JSON and H5 files.\"\"\"\n",
     "    model_json = model.to_json()\n",
     "    with open(name + \".json\", \"w\") as json_file:\n",