[bug-fix] Set number of threads based on allocated CPU count in Docker containers (#4471)

* Set num threads properly for Docker

* Pylint-friendly logic

* Use f.read().rstrip()

* Change function names

ml-agents/mlagents/torch_utils/torch.py

 import os
 
+from mlagents.torch_utils import cpu_utils
+
 # Detect availability of torch package here.
 # NOTE: this try/except is temporary until torch is required for ML-Agents.
 try:
 
-    torch.set_num_interop_threads(2)
+    torch.set_num_threads(cpu_utils.get_num_threads_to_use())
     os.environ["KMP_BLOCKTIME"] = "0"
 
     # Known PyLint compatibility with PyTorch https://github.com/pytorch/pytorch/issues/701

ml-agents/mlagents/torch_utils/cpu_utils.py

+from typing import Optional
+
+import os
+
+
+def get_num_threads_to_use() -> Optional[int]:
+    """
+    Gets the number of threads to use. For most problems, 4 is all you
+    need, but for smaller machines, we'd like to scale to less than that.
+    By default, PyTorch uses 1/2 of the available cores.
+    """
+    num_cpus = _get_num_available_cpus()
+    return max(min(num_cpus // 2, 4), 1) if num_cpus is not None else None
+
+
+def _get_num_available_cpus() -> Optional[int]:
+    """
+    Returns number of CPUs using cgroups if possible. This accounts
+    for Docker containers that are limited in cores.
+    """
+    period = _read_in_integer_file("/sys/fs/cgroup/cpu/cpu.cfs_period_us")
+    quota = _read_in_integer_file("/sys/fs/cgroup/cpu/cpu.cfs_quota_us")
+    if period > 0 and quota > 0:
+        return int(quota // period)
+    else:
+        return os.cpu_count()
+
+
+def _read_in_integer_file(filename: str) -> int:
+    try:
+        with open(filename) as f:
+            return int(f.read().rstrip())
+    except FileNotFoundError:
+        return -1