feat: Add a functional API for optimization

[engintoklu]

This pull request introduces an alternative API for EvoTorch that conforms to the functional programming paradigm. This functional API can be used together with torch.func.vmap, and therefore can be used for optimizing not just a single population, but a batch of populations simultaneously.

The main improvements are:

• Functional counterparts of cross entropy method (cem) and policy gradients with parameter-based exploration (pgpe) are implemented. These algorithm implementations can be used with vmap, or can be given batches of starting points (center_init arguments) so that they will generate batches of populations centered around them.
• Functional counterparts of gradient-based optimizers adam, clipup, and sgd are implemented. The interfaces of these optimizers are similar to the interfaces of functional cem and pgpe. Therefore, the user can switch back and forth between evolutionary approach and gradient-based approach for solving a problem, with minimum amount of code change.
• The decorator @expects_ndim is introduced. This decorator is used for declaring how many dimensions are expected per each positional argument of the decorated function. Upon receiving tensors whose number of dimensions are more than expected, the decorated function interprets those tensors as batched arguments, applies vmap on itself, and performs its operations across the batch dimensions.
• The decorator @rowwise is introduced. This decorator is used for declaring that a function is implemented with the assumption that its argument is a vector. If it receives a tensor with 2 or more dimensions, the function applies vmap on itself, and performs its operations across multiple batch dimensions.

[codecov[bot]]

Codecov Report

Attention: 113 lines in your changes are missing coverage. Please review.

Comparison is base (5c58566) 76.74% compared to head (8224905) 77.36%.

Files	Patch %	Lines
src/evotorch/distributions.py	80.79%	34 Missing :warning:
src/evotorch/core.py	31.42%	24 Missing :warning:
src/evotorch/algorithms/functional/misc.py	67.50%	13 Missing :warning:
src/evotorch/tools/misc.py	78.43%	11 Missing :warning:
src/evotorch/algorithms/functional/funcpgpe.py	89.53%	9 Missing :warning:
src/evotorch/algorithms/functional/funccem.py	87.50%	8 Missing :warning:
src/evotorch/tools/constraints.py	87.30%	8 Missing :warning:
src/evotorch/algorithms/functional/funcclipup.py	92.50%	3 Missing :warning:
src/evotorch/decorators.py	96.25%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #98 +/- ## ========================================== + Coverage 76.74% 77.36% +0.61% ========================================== Files 49 57 +8 Lines 7509 8213 +704 ========================================== + Hits 5763 6354 +591 - Misses 1746 1859 +113 ```

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