Fix replay buffer compatibility with mujoco envs

[vwxyzjn]

The current DDPG SAC TD3 files are not compatible with mujoco envs (see below), and this PR fixes it.

(cleanrl-ghSZGHE3-py3.9) ➜  cleanrl git:(fix-mujoco-compatibility) ✗ python -i ddpg_continuous_action.py --gym-id Hopper-v2 --learning-starts 100
/home/costa/.cache/pypoetry/virtualenvs/cleanrl-ghSZGHE3-py3.9/lib/python3.9/site-packages/gym/envs/registration.py:479: UserWarning: WARN: The environment Hopper-v2 is out of date. You should consider upgrading to version `v3` with the environment ID `Hopper-v3`.
  logger.warn(
global_step=23, episode_reward=8.566108703613281
global_step=41, episode_reward=7.716689109802246
global_step=63, episode_reward=17.882747650146484
global_step=73, episode_reward=6.347293853759766
global_step=86, episode_reward=10.202958106994629
global_step=99, episode_reward=7.710036277770996
Traceback (most recent call last):
  File "/home/costa/Documents/go/src/github.com/cleanrl/cleanrl/ddpg_continuous_action.py", line 200, in <module>
    next_state_actions = (target_actor.forward(data.next_observations)).clamp(
  File "/home/costa/Documents/go/src/github.com/cleanrl/cleanrl/ddpg_continuous_action.py", line 107, in forward
    x = F.relu(self.fc1(x))
  File "/home/costa/.cache/pypoetry/virtualenvs/cleanrl-ghSZGHE3-py3.9/lib/python3.9/site-packages/torch/nn/modules/module.py", line 1102, in _call_impl
    return forward_call(*input, **kwargs)
  File "/home/costa/.cache/pypoetry/virtualenvs/cleanrl-ghSZGHE3-py3.9/lib/python3.9/site-packages/torch/nn/modules/linear.py", line 103, in forward
    return F.linear(input, self.weight, self.bias)
  File "/home/costa/.cache/pypoetry/virtualenvs/cleanrl-ghSZGHE3-py3.9/lib/python3.9/site-packages/torch/nn/functional.py", line 1848, in linear
    return torch._C._nn.linear(input, weight, bias)
RuntimeError: expected scalar type Float but found Double

[gitpod-io]