Have some unit tests re-run using Torch-1.13 in the CI.

scripts/run_tests.sh

@@ -114,6 +114,7 @@ run_declearn_tests() {
     commands=(
         "run_unit_tests $@"
         "run_integration_tests $@"
+        "run_torch13_tests $@"
     )
     run_commands "declearn test suite" "${commands[@]}"
     status=$?
@@ -152,6 +153,33 @@ run_integration_tests() {
 }
 
 
+run_torch13_tests() {
+    : '
+    Verbosely run Torch 1.13-specific unit tests.
+
+    Install Torch 1.13 at the start of this function, and re-install
+    torch >=2.0 at the end of it, together with its co-dependencies.
+    '
+    echo "Re-installing torch 1.13 and its co-dependencies."
+    pip install .[torch1]
+    if [[ $? -eq 0 ]]; then
+        echo "Running unit tests for torch 1.13."
+        command="pytest $@
+            --cov --cov-append --cov-report=
+            test/model/test_torch.py
+        "
+        run_command $command
+        status=$?
+    else
+        echo "\e[31mSkipping tests as installation failed.\e[0m"
+        status=1
+    fi
+    echo "Re-installing torch 2.X and its co-dependencies."
+    pip install .[torch2]
+    return $status
+}
+
+
 main() {
     if [[ $# -eq 0 ]]; then
         echo "Missing required positional argument."