Fixes training UR10 reach with RL_GAMES and SKRL (#678)

# Description

- Fixes https://github.com/isaac-sim/IsaacLab/issues/541
- **Bug description:** UR10 reach can be trained with RSL_RL but not
with RL_GAMES and SKRL.
- **Why this problem occurs:** The actual problem is there is no
`__init__.py` file in the `agents` folder inside ur_10. Therefore when
`mod_path = os.path.dirname(importlib.import_module(mod_name).__file__)`
line is executed it gives error as
`importlib.import_module(mod_name).__file__` is not able to find the
module
`omni.isaac.lab_tasks.manager_based.manipulation.reach.config.ur_10.agents`.

## Fixes
- `__init__.py` file is added inside the ur_10.agents folder.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

source/extensions/omni.isaac.lab_tasks/omni/isaac/lab_tasks/manager_based/manipulation/reach/config/ur_10/agents/__init__.py

+# Copyright (c) 2022-2024, The Isaac Lab Project Developers.
+# All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+
+from . import rsl_rl_ppo_cfg  # noqa: F401, F403