Support for conditional sampling

[isurulucky]

Hi,

I tried out $subject and the implementation can be found in the PR #344.

I saw that there is already an issue created to change sampling implementation to top down, just sending this PR in case there is interest in using it till top down sampling is available. Would be happy to receive any suggestions for improvements. Planning to send a separate PR with some units tests, in case this is merged.

Code snippet for trying out conditional sampling:

    height = 28
    width = 28
    image_scope = np.array(range(height * width)).reshape(height, width)
    marginalize_idx = list(image_scope[0:round(height / 2), :].reshape(-1))
    vars_to_marginalize = Scope(marginalize_idx)
    sampling_query = SamplingQuery(cct)

    samples = sampling_query(x=input_to_condition.unsqueeze(1), integrate_vars=vars_to_marginalize)
    leaf_samples = samples[0]
    leaf_samples = leaf_samples[:, 0, :]

Thanks and regards,
Isuru