np.product is deprecated. Use np.prod instead.

**Describe the bug**
```
monai/networks/nets/regressor.py:146:32: error: Module has no attribute "product"  [attr-defined]
monai/networks/nets/varautoencoder.py:123:27: error: Module has no attribute "product"  [attr-defined]
```
np.product is deprecated. Use np.prod instead. https://github.com/numpy/numpy/pull/23314