Make ShapeTracker Immutable (#1909)

* ugh

* ops test pass

* fix shapetracker tests

* sym shapetracker

* shapetracker is a tuple of views now

* from_shape

* fix has variable shape

* key isn't needed

* post init assert

docs/abstractions.py

@@ -326,22 +326,22 @@ void E_1(float* data0) {
 from tinygrad.shape.shapetracker import ShapeTracker
 
 # create a virtual (10, 10) Tensor. this is just a shape, there's no actual tensor
-a = ShapeTracker((10, 10))
+a = ShapeTracker.from_shape((10, 10))
 
 # you'll see it has one view. the (10, 1 are the strides)
 print(a) # ShapeTracker(shape=(10, 10), views=[View((10, 10), (10, 1), 0)])
 
 # we can permute it, and the strides change
-a.permute((1,0))
+a = a.permute((1,0))
 print(a) # ShapeTracker(shape=(10, 10), views=[View((10, 10), (1, 10), 0)])
 
 # we can then reshape it, and the strides change again
 # note how the permute stays applied
-a.reshape((5,2,5,2))
+a = a.reshape((5,2,5,2))
 print(a) # ShapeTracker(shape=(5, 2, 5, 2), views=[View((5, 2, 5, 2), (2, 1, 20, 10), 0)])
 
 # now, if we were to reshape it to a (100,) shape tensor, we have to create a second view
-a.reshape((100,))
+a = a.reshape((100,))
 print(a) # ShapeTracker(shape=(100,), views=[
          #   View((5, 2, 5, 2), (2, 1, 20, 10), 0),
          #   View((100,), (1,), 0)])
@@ -352,7 +352,7 @@ idx, _ = a.expr_idxs()
 print(idx.render())  # (((idx0%10)*10)+(idx0//10))
 
 # of course, if we reshape it back, the indexes get simple again
-a.reshape((10,10))
+a = a.reshape((10,10))
 idx, _ = a.expr_idxs()
 print(idx.render())  # ((idx1*10)+idx0)
 
@@ -362,11 +362,11 @@ print(a) # ShapeTracker(shape=(10, 10), views=[
          #   View((10, 10), (10, 1), 0)])
 
 # ...until we simplify it!
-a.simplify()
+a = a.simplify()
 print(a) # ShapeTracker(shape=(10, 10), views=[View((10, 10), (1, 10), 0)])
 
 # and now we permute it back
-a.permute((1,0))
+a = a.permute((1,0))
 print(a) # ShapeTracker(shape=(10, 10), views=[View((10, 10), (10, 1), 0)])
 
 # and it's even contiguous