[MNT] Fixes linting issues in base, networks, registry modules (#…

…4310)

Towards #4182 

Fixes linting issues in `base`, `networks`, and `registry` modules:

- Fix `B028 No explicit stacklevel keyword argument found`

sktime/base/_base.py

@@ -840,7 +840,7 @@ def _deprecate_tag_warn(cls, tags):
                     )
                 else:
                     msg += ', please remove code that access or sets "{tag_name}"'
-                warnings.warn(msg, category=DeprecationWarning)
+                warnings.warn(msg, category=DeprecationWarning, stacklevel=2)
 
 
 class BaseEstimator(BaseObject):

sktime/networks/lstmfcn_layers.py

@@ -642,7 +642,8 @@ def __init__(
                 warnings.warn(
                     "`implementation=0` has been deprecated, "
                     "and now defaults to `implementation=1`."
-                    "Please update your layer call."
+                    "Please update your layer call.",
+                    stacklevel=2,
                 )
                 implementation = 1
 
@@ -653,7 +654,8 @@ def __init__(
                         "when using dynamic RNNs (i.e. non-unrolled). "
                         "You can either set `unroll=True`, "
                         "set `dropout` and `recurrent_dropout` to 0, "
-                        "or use a different backend."
+                        "or use a different backend.",
+                        stacklevel=2,
                     )
                     dropout = 0.0
                     recurrent_dropout = 0.0

sktime/registry/_lookup.py

@@ -245,7 +245,7 @@ def _is_ignored_module(module):
                 # Skip missing soft dependencies
                 if "soft dependency" not in str(e):
                     raise e
-                warnings.warn(str(e), ImportWarning)
+                warnings.warn(str(e), ImportWarning, stacklevel=2)
 
     # Drop duplicates
     all_estimators = set(all_estimators)