Raise error on constructing an array from mixed type inputs

[galipremsagar]

Description

We currently are allowing construction of mixed-dtype by type-casting them into a common type as below:

In [1]: import cudf

In [2]: import pandas as pd

In [3]: s = pd.Series([1, 2, 3], dtype='datetime64[ns]')


In [5]: p = pd.Series([10, 11])

In [6]: new_s = pd.concat([s, p])

In [7]: new_s
Out[7]: 
0    1970-01-01 00:00:00.000000001
1    1970-01-01 00:00:00.000000002
2    1970-01-01 00:00:00.000000003
0                               10
1                               11
dtype: object

In [8]: cudf.Series(new_s)
Out[8]: 
0   1970-01-01 00:00:00.000000
1   1970-01-01 00:00:00.000000
2   1970-01-01 00:00:00.000000
0   1970-01-01 00:00:00.000010
1   1970-01-01 00:00:00.000011
dtype: datetime64[us]

This behavior is incorrect and we are getting this from pa.array constructor. This PR ensures we do proper handling around such cases and raise an error.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

One suggestion for improvement.

[galipremsagar]

/merge