clarify and update import style

[mvesin]

As a developer want a clear and homogeneous import style in the library

• document in CODING_RULES.md styles A. and B. are the prefered coding style for imports + remind import order
• update the library to match these styles (cases 1. 2. 3. except optimizers.declearn that seems to be purposely written as a submodule)

See details below

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We currently have a lack of homogeneity in the way we do imports from fedbiomed.

In the majority of cases we do:

A. when importing from a different module (including a submodule) use full path and "public" file names (not starting with _) to use the module's __init__.py eg

# a file not in the `logger` module
from fedbiomed.common.logger import logger

B. when importing from same module, use relative path and private file eg

# a file in fedbiomed.common.data
from ._torch_data_manager import TorchDataManager

Styles A. and B. look fine IMO, but we maybe lack a written trace in CODING_RULES.md

The thing is that we sometimes have different import styles:

1. using full path for import from same module eg

# a file in fedbiomed.researcher.federated_workflows
from fedbiomed.researcher.federated_workflows._federated_workflow \
    import exp_exceptions, FederatedWorkflow

2. using public files for module files when we probably intend to use private file (aggregator, strategy) eg

# in strategy/__init__.py => config to import as strategy.Strategy
__all__ = [
    "DefaultStrategy",
    "Strategy",
]
# but file is named strategy/strategy.py to import as strategy.strategy.Strategy

... and we use sometimes as submodules, sometimes not.

3. specific case of fedbiomed.common.optimizers. Seems to be a mix of intended submodule (declearn.py) and un intended one (generic_optimizers.py optimizers.py)

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Also regarding import orders we should always use:

1 - builtins
2 - third party
3 - from library source (from fedbiomed)
4 - and from module directory like from ._module