When I use torch.cov to compute the covariance matrix of a batch of vector (in which the batch_size may be less than the vector length,) and then use the statistics to construct the MultivariateNormal distribution, it would raise a ValueError which indicates the covariance_matrix is not a positive-definite matrix.

```
import torch
from torch.distributions.multivariate_normal import MultivariateNormal
data = torch.rand(2,4)
data_mean = data.mean(0)
data_cov = torch.cov(data.T)
m = MultivariateNormal(data_mean, data_cov)
```

When data (NxD) with N<D, this would happen since the covariance_matrix may be positive-semidefinite. I found that in NumPy, the MultivariateNormal distribution only requires positive-semidefinite but PyTorch need positive-definite covariance_matrix.

To achieve my goal, I need to transform the covariance_matrix to the numpy format for random sampling and then transform the sampled results back into PyTorch tensor.