ValueError: expected sequence of length 3 at dim 1 (got 2)

This error occurs when a function or operation expects a sequence (such as a list or array) of length 3 at dimension 1, but instead receives a sequence of length 2 at that dimension.

For example, if you have a function that expects a 3D array (i.e. an array with three dimensions), and you pass in an array that is missing a dimension, you will get this error.

Here is an example of code that could produce this error:

import numpy as np

# Create a 2D array
arr = np.array([[1, 2], [3, 4], [5, 6]])

# Reshape the array to be 3D
arr_3d = arr.reshape((3, 2, 1))

# Try to perform an operation on the array that expects 3 dimensions
np.mean(arr_3d, axis=1)

In this case, the `np.mean` function expects a 3D array, but `arr_3d` only has 2 dimensions. This results in the error message:

ValueError: expected sequence of length 3 at dim 1 (got 2)

To fix this error, you need to make sure that the sequence you are passing in has the expected length at dimension 1. In the example above, you could fix the error by reshaping `arr` to have three dimensions instead of two:

import numpy as np

# Create a 2D array
arr = np.array([[1, 2], [3, 4], [5, 6]])

# Reshape the array to be 3D
arr_3d = arr.reshape((3, 1, 2))

# Perform the operation on the 3D array
np.mean(arr_3d, axis=1)