code: import numpy as np
  
  from ray.rllib.utils import try_import_tree
  from ray.rllib.utils.framework import try_import_torch
  
  torch, _ = try_import_torch()
  tree = try_import_tree()
  
  
  def explained_variance(y, pred):
      y_var = torch.var(y, dim=[0])
      diff_var = torch.var(y - pred, dim=[0])
      min_ = torch.Tensor([-1.0])
      return torch.max(
          min_.to(device=torch.device("cuda"))
          if torch.cuda.is_available() else min_,
          1 - (diff_var / y_var))
  
  
  def global_norm(tensors):
      """Returns the global L2 norm over a list of tensors.
  
      output = sqrt(SUM(t ** 2 for t in tensors)),
          where SUM reduces over all tensors and over all elements in tensors.
  
      Args:
          tensors (List[torch.Tensor]): The list of tensors to calculate the
              global norm over.
      """
      # List of single tensors' L2 norms: SQRT(SUM(xi^2)) over all xi in tensor.
      single_l2s = [
          torch.pow(torch.sum(torch.pow(t, 2.0)), 0.5) for t in tensors
      ]
      # Compute global norm from all single tensors' L2 norms.
      return torch.pow(sum(torch.pow(l2, 2.0) for l2 in single_l2s), 0.5)
  
  
  def huber_loss(x, delta=1.0):
      """Reference: https://en.wikipedia.org/wiki/Huber_loss"""
      return torch.where(
          torch.abs(x) < delta,
          torch.pow(x, 2.0) * 0.5, delta * (torch.abs(x) - 0.5 * delta))
  
  
  def l2_loss(x):
      """Computes half the L2 norm of a tensor without the sqrt.
  
      output = sum(x ** 2) / 2
      """
      return torch.sum(torch.pow(x, 2.0)) / 2.0
  
  
  def reduce_mean_ignore_inf(x, axis):
      """Same as torch.mean() but ignores -inf values."""
      mask = torch.ne(x, float("-inf"))
      x_zeroed = torch.where(mask, x, torch.zeros_like(x))
      return torch.sum(x_zeroed, axis) / torch.sum(mask.float(), axis)
  
  
  def minimize_and_clip(optimizer, clip_val=10):
      """Clips gradients found in `optimizer.param_groups` to given value.
  
      Ensures the norm of the gradients for each variable is clipped to
      `clip_val`
      """
      for param_group in optimizer.param_groups:
          for p in param_group["params"]:
              if p.grad is not None:
                  torch.nn.utils.clip_grad_norm_(p.grad, clip_val)
  
  
  def sequence_mask(lengths, maxlen=None, dtype=None):
      """Offers same behavior as tf.sequence_mask for torch.
  
      Thanks to Dimitris Papatheodorou
      (https://discuss.pytorch.org/t/pytorch-equivalent-for-tf-sequence-mask/
      39036).
      """
      if maxlen is None:
          maxlen = lengths.max()
  
      mask = ~(torch.ones((len(lengths), maxlen)).to(
          lengths.device).cumsum(dim=1).t() > lengths).t()
      mask.type(dtype or torch.bool)
  
      return mask
  
  
  def convert_to_non_torch_type(stats):
      """Converts values in `stats` to non-Tensor numpy or python types.
  
      Args:
          stats (any): Any (possibly nested) struct, the values in which will be
              converted and returned as a new struct with all torch tensors
              being converted to numpy types.
  
      Returns:
          Any: A new struct with the same structure as `stats`, but with all
              values converted to non-torch Tensor types.
      """
  
      # The mapping function used to numpyize torch Tensors.
      def mapping(item):
          if isinstance(item, torch.Tensor):
              return item.cpu().item() if len(item.size()) == 0 else \
                  item.cpu().detach().numpy()
          else:
              return item
  
      return tree.map_structure(mapping, stats)
  
  
  def convert_to_torch_tensor(stats, device=None):
      """Converts any struct to torch.Tensors.
  
      stats (any): Any (possibly nested) struct, the values in which will be
          converted and returned as a new struct with all leaves converted
          to torch tensors.
  
      Returns:
          Any: A new struct with the same structure as `stats`, but with all
              values converted to torch Tensor types.
      """
  
      def mapping(item):
          if torch.is_tensor(item):
              return item if device is None else item.to(device)
          tensor = torch.from_numpy(np.asarray(item))
          # Floatify all float64 tensors.
          if tensor.dtype == torch.double:
              tensor = tensor.float()
          return tensor if device is None else tensor.to(device)
  
      return tree.map_structure(mapping, stats)
  
  
  def atanh(x):
      return 0.5 * torch.log((1 + x) / (1 - x))
  
apis: numpy.asarray