Restore pre-4625 way how the `size` kwarg works

[michaelosthege]

Due to a (big) misunderstanding, my #4625 PR broke the way how size parametrization works in v4.

The misunderstanding was specifically about the notion of implied dimensions and what it means for univariate vs. multivariate RVs:

⚠ The pre-#4625 notion was that size is in addition to the support dimensions.

⚠ The post-#4625 notion was that size is in addition to what's implied by parameters other than size/shape/dims.

The difference is subtle and maybe best explained with the following example:

# v4 pre-4625 (45cb4ebf36500e502481bdced6980dd9e630acca)
MvNormal.dist(
    cov=eye(7), mu=ones(7), size=(2,3)    # MvNormal is multivariate with ndim_support=1,
).eval().shape == (2, 3, 7)               # therefore (7,) is implied/required and does not count into `size`.
Normal.dist(
    mu=[1,2,3], size=(2,3)
).eval().shape == (2,3)                   # Normal is univariate, so `mu` does not count as a support dim

# v4 post-4625 (e9f2e9616394275ccf7587a4818fe21251d51328)
MvNormal.dist(
    cov=eye(7), mu=ones(7), size=(2,3)
).eval().shape == (2, 3, 7)
Normal.dist(
    mu=[1,2,3], size=(2,3)
).eval().shape == (2, 3, 3)         # the last dimension of length 3 was implied by mu and does not count into `size`

With the changes from #4625 the outcome from specifying shape=(1,2,3, ...) and size=(1,2,3) is identical.

After some discussion about the advantages/disadvantages of either API flavor, we decided to go back to the pre-4625 flavor where size is essentially shape but without support dimensions.
This is also the way how numpy handles dimensionality of multivariate distributions:

np.random.mvnormal(
    cov=np.eye(7), mean=np.ones(7), size=(2, 3)
).shape == (2, 3, 7)

The flexibility added by #4625, namely the ability to not specify dimensions that are implied from RV support or parameters, will continue to work through the shape with Ellipsis API:

mu = aesara.shared([1, 2, 3])

rv = Normal.dist(
    mu=mu, shape=(7, 5, ...)   # only the additional dimensions are specified explicitly
)
assert rv.eval().shape == (7, 5, 3)

# Now change the parameter-implied dimensions:
mu.set_value([1, 2, 3, 4])
assert rv.eval().shape == (7, 5, 4)

[twiecki]

_____________________________ test_logp_scalar_ode _____________________________

    @pytest.mark.xfail(
        condition=sys.platform == "win32", reason="See https://github.com/pymc-devs/pymc3/issues/4652."
    )
    def test_logp_scalar_ode():
        """Test the computation of the log probability for these models"""
    
        # Differential equation
        def system_1(y, t, p):
            return np.exp(-t) - p[0] * y[0]
    
        # Parameters and inital condition
        alpha = 0.4
        y0 = 0.0
        times = np.arange(0.5, 8, 0.5)
    
        yobs = np.array(
            [0.30, 0.56, 0.51, 0.55, 0.47, 0.42, 0.38, 0.30, 0.26, 0.21, 0.22, 0.13, 0.13, 0.09, 0.09]
        )[:, np.newaxis]
    
        ode_model = DifferentialEquation(func=system_1, t0=0, times=times, n_theta=1, n_states=1)
    
        integrated_solution, *_ = ode_model._simulate([y0], [alpha])
    
        assert integrated_solution.shape == yobs.shape
    
        # compare automatic and manual logp values
        manual_logp = norm.logpdf(x=np.ravel(yobs), loc=np.ravel(integrated_solution), scale=1).sum()
        with pm.Model() as model_1:
            forward = ode_model(theta=[alpha], y0=[y0])
            y = pm.Normal("y", mu=forward, sd=1, observed=yobs)
>       pymc3_logp = model_1.logp()
     new_var = var.type.filter_variable(new_var, allow_convert=True)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = TensorType(float32, col), other = Rebroadcast{?,0}.0
allow_convert = True

    def filter_variable(self, other, allow_convert=True):
        """
        Convert a symbolic Variable into a TensorType, if compatible.
    
        For the moment, only a TensorType and GpuArrayType will be
        converted, provided they have the same number of dimensions
        and dtype and have "compatible" broadcastable pattern.
    
        """
        if not isinstance(other, Variable):
            # The value is not a Variable: we cast it into
            # a Constant of the appropriate Type.
            other = self.Constant(type=self, data=other)
    
        if other.type == self:
            return other
    
        if allow_convert:
            # Attempt safe broadcast conversion.
            other2 = self.convert_variable(other)
            if other2 is not None and other2.type == self:
                return other2
        raise TypeError(
>           f"Cannot convert Type {other.type} "
            f"(of Variable {other}) into Type {self}. "
            f"You can try to manually convert {other} into a {self}."
        )
E       TypeError: Cannot convert Type TensorType(float32, matrix) (of Variable Rebroadcast{?,0}.0) into Type TensorType(float32, col). You can try to manually convert Rebroadcast{?,0}.0 into a TensorType(float32, col).

[michaelosthege]

@twiecki that's issue #4652 . Any insight is greatly appreciated.

[michaelosthege]

Closed by #4693