Fix tensorboad video slow numpy->torch conversion

[NickLucche]

Hey,

this PR is a small docs fix to the speed issue highlighted here #196 (comment), which also shows up as a warning now in more recent torch versions
:UserWarning: Creating a tensor from a list of numpy.ndarrays is extremely slow. Please consider converting the list to a single numpy.ndarray with numpy.array() before converting to a tensor. (Triggered internally at ../torch/csrc/utils/tensor_new.cpp:275.).

You can also test the speed difference yourself with:

from time import time
import numpy as np
import torch

rgb = np.ones((3,256,320))
rgbs = [rgb.copy() for _ in range(100)]

s = time()
a = torch.ByteTensor(rgbs)
print(time()-s)

s = time()
a = torch.from_numpy(np.asarray(rgbs))
print(time()-s)

I get two orders of magnitude of difference in times here.

Description

I create a numpy array first and then let torch use the same storage. np.stack could also be suggested if we prefer being more explicit.

Motivation and Context

Fixes #196 (comment).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have opened an associated PR on the SB3-Contrib repository (if necessary)
• I have opened an associated PR on the RL-Zoo3 repository (if necessary)
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)
• I have checked that the documentation builds using make doc (required)

Note: You can run most of the checks using make commit-checks.

Note: we are using a maximum length of 127 characters per line

[araffin]

LGTM, thanks =)