One Thread or Process Question

Hello! I suffer from some issues when I use the 'train' function of the strategy. I try to use different strategies, such as GEM or Naive, but I still suffer from this problem.

The reported error is shown as follows:

```
Exception in thread Thread-1:
Traceback (most recent call last):
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/site-packages/psutil/_common.py", line 443, in wrapper
ret = self._cache[fun]
AttributeError: 'Process' object has no attribute '_cache'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/site-packages/psutil/_pslinux.py", line 1645, in wrapper
return fun(self, *args, **kwargs)
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/site-packages/psutil/_pslinux.py", line 1877, in memory_info
with open_binary("%s/%s/statm" % (self._procfs_path, self.pid)) as f:
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/site-packages/psutil/_common.py", line 728, in open_binary
return open(fname, "rb", buffering=FILE_READ_BUFFER_SIZE)
FileNotFoundError: [Errno 2] No such file or directory: '/proc/78345/statm'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/threading.py", line 932, in _bootstrap_inner
self.run()
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/threading.py", line 870, in run
self._target(*self._args, **self._kwargs)
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/site-packages/avalanche/evaluation/metrics/ram_usage.py", line 74, in _f
ram_usage = self._process_handle.memory_info().rss / 1024 / 1024
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/site-packages/psutil/_common.py", line 446, in wrapper
return fun(self)
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/site-packages/psutil/__init__.py", line 1058, in memory_info
return self._proc.memory_info()
File "/data/miniconda3/envs/env-3.8.8/lib/python3.8/site-packages/psutil/_pslinux.py", line 1652, in wrapper
raise NoSuchProcess(self.pid, self._name)
psutil.NoSuchProcess: process no longer exists (pid=78345)
```

Thank you very much!